The things that make this game great are so great that the problems it has just won't matter.
The Story is amazing. It sucks you in once you give it a chance (I say that because your first mission on Eden Prime just isn't the most engaging, for me the game started to be fascinating just after I reached the Citadel and started to understand the History, the characters and the alien races).
The more you play the more you just can't put your controller down! It stays with you even when you're not playing, as you do your daily business you keep thinking about the characters and the story.
One other thing I found to be very well done in the game was the Art Design. It sometimes looks a little blurry, but you get the feeling that this mist that covers the air in the universe of Mass Effect is just part of this artistic design. It gives the idea of a more intense and mysterious world, that holds a lot more that you may expect.
Now let me head for the points of failure. Although this game is fantastic and inspired, it has some setbacks. As I was saying before, the first mission (on Eden Prime) isn't the most compelling, and some of the other missions have the same problem. Story-driven missions usually won't disappoint you, I just found that some of them have this really tedious beginning, although every one of them pick up at the middle or ending (even Eden Prime). But they aren't really action-packed and they don't look that great... As it is with Feros, although the ending of the level is just fantastic I had a problem with it because it was boring, mainly because the level was very repetitive.
Talking about repetition, the non-story missions had a LOT of it. They all happened in the same old warehouse in some random planet, and to get to it you had to use the Mako (and you also use it for most of the story-driven missions too, which isn't bad... It just isn't fun either). Those planets also had some collectibles you could find by investigating this anomalies in the surface.
Other than repetition, boring elevator sequences (how I HATE to enter an elevator in this game... If you played it you know what I'm talking about), and some visual hiccups (framerate is not stable at all! But it isn't enough to affect the gameplay) the game is fantastic. The setbacks are somewhat serious but the story and action just are too darn good! It turns out that any problems this game might have, they aren't enough to detract you from the "amazingness" of it!!! I mean it!
Conclusion: a MUST buy! And it is the best time to do it... It doesn't cost much anymore and you can even buy it on the Game Marketplace at Xbox Live.
Final Opinion
Best point of the game: The encounter with Vigil. When it happens, you'll love it! (if you play a game for the Story of course, as I'm guessing you do, since its and RPG) I'm not gonna say anything about it 'cause it would spoil it... But believe me, its great!
Worst point of the game: The side-missions on uncharted planets. The ones on the Citadel and elsewhere are great though!