Saw it yesterday. Quite enjoyed it (as I did the first two movies in the series). I will definitely buy the DVD when it's released.
I've never really been into the Resident Evil games in a big way, just played a couple of them casually, so any inaccuracies in the movies to the source material doesn't bother me in the slightest.
I view these more as zombie/monster movies, rather than Resident Evil game movies.
I love post-apocalyptic movies and zombie movies, so Extinction was right up my street. While no means Oscar material, it was overall a decent movie I thought.
Graham