Although Mass Effect suffers from a number of significant flaws, it is overall one of the most enjoyable roleplaying games I've played in some time.
Mass Effect's greatest strength is the quality of the writing and acting. When you strip the story down to its basic underpinnings, it's nothing we haven't seen many times before - but the masterful execution of the plot, the absolutely top-notch writing and dialogue, and the high-quality performances of virtually every voice actor in the game make it a pleasure to watch the story unfold. It's rare to see such well-written dialogue in an RPG. The backdrop of the story is well above par, as well - it's clear that a great deal of time and effort went into developing a detailed and fairly credible setting for the game, though it would have been nice to be able to explore more of the colonized planets.
Unfortunately, even though the main storyline is very well-written, the sidequests are often brief, uninspiring and not very detailed, and your six companions generally don't say much during missions and are not very well developed (though they're at least fun to have around). Oddly enough, the best characterization goes to Shepard, the mostly-blank-slate main character. Your ethical choices are more about law vs. chaos than good vs. evil, which is an interesting take on traditional computer roleplaying. Regardless of the moral path you choose, Shepard's lines and voice acting are always convincing and never dull (though I played as a female Shepard, so I can't speak for the male version).
Combat is simple, but reasonably enjoyable. It's played out as a third-person shooter, with a pause menu for selecting special abilities for Shepard or your teammates (two of whom will join you at any given time). You level up with experience gained from combat and quest completion, earning skill points to improve your aim, endurance, et cetera.
Overall, Mass Effect is a flawed but extremely engrossing title. The lopsided gameplay and weak sidequests are more than compensated for by the engaging dialogue and well-written storyline. It's arguably short - it took me a bit more than 30 hours to complete the game, but I did all the sidequests I came across, which many people probably won't have the patience for - but at $20, it's well worth the price. The only thing to be aware of is that the game does require online activation with EA, though a revoke tool to reclaim your activations is available.