If a game is offering up great atmosphere and an interesting world and/or story, I can forgive quite a bit with regards to gameplay. I love the Silent Hill games, might be my favorite series, but the gameplay is absolutely nothing special at all.
But then, I'm the guy who likes to call every person on the Codec over and over and over in a MGS game just to make sure I've heard everything they have to say. I love great gameplay, but I also appreciate great production value when I can get it and if Xenosaga wants to launch into a 20 minute cinema, well... I'll just switch my brain to "watch" mode and enjoy it while it lasts.
Ideally you get something like God of War, which can do both, and even merge the two. But not every game can be God of War.
Also, for the record, I hate Halo, thought KotOR was just okay, and I love Half Life and am currently creaming my pants over it as I type this.
Also Mass Effect looks great.