This was the first movie in the series to be released in 3D. Although the 3D could have been utilized more (although that would come across as being ridiculous in 2D), the movie itself wasn't a terrible entry into the series. The good thing about the Resident Evil movies is they don't all follow the exact same story ideas.
Of course there's the central idea of the movie being the zombie creatures and there's this evil corporation and such, but the movies themselves are all different. The first is a kind of almost futuristic zombie movie in an underground complex. The second is this kind of die hard, quaranteened action movie. The third is a bit more of a wild-west zombie fare. You follow?
So the third, again, has a new setting, a new group of characters (plus some returning characters), and some new monsters. In this installment, Alice has gone on to find that kind of utopia safe area (forget what it's called) in Alaska, and finds nothing. So she returns to LA with that chick from the third movie, and lands the plane on the roof of a prison. The prison is surrounded by zombies. So what're they do? Of course they get Wentworth Miller in to help them escape (because if you're gonna break out of prison, you need him on your side).
I mean it won't win any acadamy awards, but it is a decent movie if you enjoyed the previous entries.