On the whole I love it and I think it's an excellent rules alternative to regular D20, but I don't think it'll be for everyone and it does create some potential issues.
 
Character generation is similar and won't throw most people off.  It's still OGL so all the D20 terms are here (mostly).  You still have your base 6 ability scores but instead of the 1 - 20 scale they are simply dealt with by the modifiers.  So you may have a STR +1 and that'd be it for your stat.  Saving throws are the same with the addition of a toughness one.  Which is actually the replacement for Hit Points.
 
No more hit points.  Instead now you have a damage track.  For anyone that's played a World of Darkness or Shadowrun game (or probably others) you'll be familiar to the concept.  Damage now is fixed, a Sword no longer does 1d8 + Strength it's not just Strength + 3 for example.
 
If you are hit you make a toughness save versus a difficulty of 15 + Damage (strength + weapon + Mods).  Your Toughness save is equal to Constitution + Armour.  If you equal or beat the difficulty you take no damage.  For every 5 points you fail the save by the type of damage you take is worse.  From brusied/hurt - unconsious/dying (non-leathal/leathal).  As your condition gets worse your damage saves get harder and you also may incure other pentalties depending on the wound (dazed, stunned).  You can take any number of brusied/hurt wounds but the rest go from bad to worse.
 
There are some ways you can shrug off damage through the use of conviction to help lessen the effects.  But ultimately it means you don't want to get hit.  I like the fact that this system really makes constitution worth while as well as the toughness feat (gives you +1 to your save each time you take it).   Hopefully this all leads to characters acting more strategically and cautiously instead of blindly rushing in.  
 
I also like the fact that it creates a debilitating effect in combat.  No longer do you fight unimpared due to injuries until you finally drop.  You start to get hurt and it gets worse from there.  
 
Additionally I like is that weapon types are more standardized.  No longer do you have to crunch the numbers in deciding between longsword/rapier - it's now just a sword.  You can be much more free to have a character use his weapon of choice.
 
Classes.  
As I mentioned there are only 3 classes.  Adept (spells), Expert(skills), Warriror(combat).  You now develop your particular flavor of class through feats alone.   You won't start out as powerfull as a stock D20 character but you can make a much more personalized class.  Also you gain feats at each level so you can create a very custom class.   Of the 3 classes there are some distinctions.  1st each class has a primary ability that you only get if you take that class at level 1.  These abilities are tied to the use of conviction and will seperate a 1/1 Adept/Warriror from a 1/1 Warriror/Adept.  They also develop their Combat Bonus, Saves at different rates, simialr to the breakdowns between stock D20 Wizard/Theif/Warriror.
 
Combat.  
Your attack roll is equal to your Base Attack Bonus from your class + Dex + misc mods.  Yes, that means that a fighter who wants to hit things can no longer just rely on strength.  The difficulty is equal to 10 + opponents BAB + Dodge/Parry (dex/strength).  So now your actual combat skill will help to determine if you get hit or not.  
 
There are feats that effect your dodge/parry and also feats that allow you to do cool tricks should you successfully dodge/parry your oppenent.  It looks like combat should be much more dynamic and flashy with less of a whack-a-mole approach to it.
 
You could probably create some more varriability into the system by changing the difficulty for your to hit roll from 10 +... to 1d20+... that your opponent rolls.  It would slow combats down, but it could make them that much more exciting also.
 
Feats and skills are largely uneffected there are some variations to the stock ones because of the mechanics but nothing really shocking here.  Though you do now get a feat at every level.  Which seams like a lot.  But if you consider classes don't get special tricks and there is no bonus feat every 3 levels it works out about the same.  Where a D20 thief would get uncanny dodge extra back stab and whatever else as you level now you'd simpley spend your feat on those.  It goes back to the original intent allowing the player build the type of character they want.  So if you'd rather have a theif that's an expert in traps/locks and pick pocketing don't waste time with putting feats into back stab.  
 
Magic.  
The magic system is very different from stock D20.  No longer do you have lists of spells that you can pick and choose from at each level and a large spell book to decide between.  It's much more similar to the psyioncs system.  You use a feat slot to learn a power.  You can then use that power as often as you want.  However most powers potentially cause fatigue so you could end up knocking yourself out through casting spells.  Spell casters will no longer be walking swiss army knives.  They will have to more carefully pick the types of effects they want to use.  Though the spells themselves often have more variation in their castings.  
 
It's hard to say how effective they spells will be in relation to the rest of the game or in comparison to regular D20.  But, in reading them they seemed balanced.  I think it also helps to keep casters functional in the game longer.  No longer do they have to worry about saving a spell for the right situation they can now cast spells as they see fitting.  However risking fatigue in the process.  
 
I think the game istelf is an overall lower key system then regular D20.  From low to high levels unless you are puting a lot of feats into Tough or getting uber equipment, combat will only serve to get more dangerous.  Your skill then, plays a much larger roll in your survival.  You don't have the raw abiltiy to soak damage through massive hit points.  For me, I see this as a huge plus to the game as I prefer the Grim & Gritty approach to the high fantasy of Regular D20.  In a modern setting this certainly feels like a more appropriate stock system.  simply because the damage system keeps the leathality of guns on par regardless of level, which is one thing I felt fell out of whack quickly in D20 Modern (D20 Deadlands was even worse).
 
There are still the random factors of the D20 itself that will crop up.  You could convert the game to 2d10 but I think you'd have to be carefull in the toughness save.  15+ may make the game more deadly then it initially appears.  Another alternative would be for the attacker to roll for damage.  
 
For example.
Currently combat looks like this; Attacker rolls to hit vs 10+opponents defense modifiers.  Then Opponent rolls damage save vs 15+attackers damage modifiers.
 
This could be changed to; Attacker rolls to hit (2d10 + mods), opponent then rolls defense (2d10+ mods)  attacker then rolls damage (2d10 +mods), opponent then rolls toughness save (2d10 + mods) vs the damage roll.
 
It would slow combat down, but for players that like to feel a little more involved in the game it could be more rewarding.   Of course could do the same with the standard D20, but that would make combat extremely varriable, especially at lower levels.
 
One area I haven't really touched on and have saved for the last because it's potentially one of the most interesting changes to the rules.  
Conviction is a new idea, it's sorta been used before like action dice in Eberron.  However this time it's fully integrated into the mechanics of True20 and doesn't feel like an addon.  Conviction allows you to do re-rolls and heal much like action dice.  But it's also used to power some feats, special powers and also your classes core abilities.  It's not something you just rely on occasionally when the going gets tough, but an integral part of the mechanics much like Fortune/Fate points are to Warhammer.
 
Every character gets a fixed ammount of Conviction points to pull from. They recover slowly over time or by acting in accordance to your Nature.  Nature is the replacement for alignment and a much more appropriate replacement there could not be.  Every character has a Virtue and Vice.  If you act in accordance to either one of them you can gain back points of conviction.  It's a great tool for the GM to use and the player to play into.  If you have a Greed as Vice and know that taking that Gem would be a bad thing to do there is no real insentive to taking the Gem (other then money).  However now if you are low on conviction and are greedy, well suddenly taking that Gem is a very character driven action and you are rewarded by acting within your nature.  It opens up some very interesting mechanics for characters and NPC's alike.
 
One final and minor note.  Because True20 is a generic system to replace both fantasy, modern and futuristic role playing there is wealth system in effect.  Use it if you like it or put in the equipment pricing from whatever source you prefer.  But the basics are you have a starting wealth level of say 20.  If you buy something that costs 3 (like a suit of armor) your wealth drops.  If you find something your wealth increases.  That's the gist of it.