Mass Effect 2 is in my top 3 games of all time. I never played the first one (being a PS3 player, we never got it), but I would love to play it.
ME2 is simply incredible. One of the most fun games I've played ever. The story, the visuals and the gameplay are the best you will find in any game. I've beaten ME2 once and about to do it again (and then I'm going on a trophy run).
As for ME3, I already have it preordered. Judging from the trailers, the AI of both your enemies and your teammates have improved. I always thought in ME2, your teammates always chose the worst spot for cover in the middle of a massive shootout.
I also read they are making it more challenging. I think they are stepping up each difficulty by 1 (I would hate to see Insane mode :/)
Bookmarks