Yes, it has a very predictable/derivative plot and yes it's very mainstream, but damn it, it was done really well. Believe it or not, I would give it a solid DS-8 and maybe 8.5. There are lots of little technical things that they do in it that make the tired plot (killer zombies chasing the intrepid soldier/heroes) seem interesting and somewhat exciting. I'm not going to bullshit you, it does have some very cheesy moments and it does get a bit predictable, but I would honestly say that it looks like the folks who made the movie where having fun with what they were doing.
I also have to give them credit for trying some interesting stuff in a mainstream movie. One technical thing that I enjoyed was how they cut scenes. Several key scenes went from expository dialogue straight to action scenes without any transition. One second two characters would be talking in a room, then next they'd be running down a hallway with a bunch of zombies right behind them.
It also helps that they movie is mostly set on a very tight time-frame (2h, 47m of movie time squeezed into about 90m of film). The action just starts up and never really eases up. This movie doesn't address any big philosophical issues (e.g. In the Bedroom which I was a couple of weeks ago with the Queen) but it also doesn't pretend to be anything else. Oh, and there is also a remarkable lack of outright gory stuff. A lot of times the really gruesome stuff is left up to your imagination with just a hint of a *squelch* or a sickening *crunch* while the violence happens off screen. Not a movie to take youngsters to, but it was actually somewhat surprising at times. It's the first movie in a while to make me cringe at the implied off-screen violence without having it be gratuitous (granted the movie itself is gratuitous, but we all knew that going into this :).
Oh, and it also flows along a lot like the actual video game does (e.g. first person shooter) and I think it works fairly well. We spend most of our time following one main character and we see lots of action sequences etc... It feels like a video game in a good way.
If you tend to enjoy action or horror movies you should definitely check this out. It has a solid soundtrack, it's very well done and it's really rather fun. it's like playing a video game without the annoying repetitive parts :)