Is this any good? How about solo?

It's basically a better Doom - if you liked Doom, you will like this.

Does it have much replay value? Does it have much depth?

Here's a review
http://boardgamegeek.com/thread/683748/gears-of-war-the-boardgame-review-with-pictures

I played it once and it was great. It is indeed a much superior version of Doom (which I've played too), with not only much better components but also a much more interesting rule set underlying it. It's super interesting and a great co-op, thanks to the interactivity of the action economy. It's much more refined and subtle than the D&D co-op games WotC has produced lately. On the down side, it lacks Doom's absurd difficulty - so if you like a challenge, you might have to hunt down some custom modules for it.

But if you like the theme at all and have 2-3 friends with whom you can see this playing regularly, you should definitely pick up this game.

As with any co-op, half the fun is the bickering about the 'best course of action' - we ended up shooting each other for laughs, while the big boss stood by, blinking idly. Any game that can transform an organized troupe of highly educated adults into an unruly mob of sulking, bickering teenagers with anger issues has surely done something right. In light of all that, I couldn't say how much is lost when playing this solo.

I'd be interested primarily in the solo component. The D&D games do look a bit lightweight. I gather GoW can be plahyed completely solo (as opposed to only a couple of missions solo and then I'm stuffed).