I'd give it a 6 outta 10. I liked it, but it took an effort on my part to overlook several moments of total crap in it. The fights and stuff were cool, the weapons were cool (somebody should have taken down at least one alien with a G36 though...), and the effects were pretty good. It just felt really really rushed (since they crammed it into about 3/4's the time of your average movie, wtf?) and not written well.
All it really needed was a quality writer/director like James Cameron again. You can just tell it wasn't written/directed well (just like Alien3 and Alien 4). Anderson just wasn't up to it....
I hope hope hope the rumors of both Cameron and Scott's involvement in Alien5 turn out to be true, could be soooooo sweet...
All that said, I'd probably buy the DVD for $10-$15, which is WAYYYY more than I can say for Alien3 or 4 (wouldn't give you 50 cents for either)...