I loved Mass Effect, but just like KOTOR and KOTOR II, once you're done, there's not much replay value, if none at all. I beaten KOTOR two times to see both endings, and KOTOR II also two times, and I beat Mass Effect three times, and at the moment Mass Effect is uninstalled, and back then both KOTORs took the dust on my desk after beating them twice.
Those types of games are interactive books/movies, it's heavily scripted, and it's alright because it needs to be, because it's based on a story written in a book (Mass Effect's case), and they couldn't make a free-roaming type of game with the story of Mass Effect because then you'd need to create dozens of planets filled with as much content as found in Fallout 3 on each of them, forget it, that'd take a 1,000-men team in a ten years development process. They had to shrink it a lot to make it a "game", or else better leave it as a book and add some pictures.
It's the same with Deus Ex, with Half-Life and any other story-driven games. They might well be triple and quadruple A games in quality AND content, but their replay value is almost always lacking, once the story has been told, once the single or multiple endings are seen, when you've heard all dialog lines and done all side quests all you have left to do it do it all over again just like when you read a book and you know exactly what's going to happen anyway, and that's not necessarily the goal of video gaming, but it's fine, because it's entertaining, for some time.