A great story and environment set in an ubiquitously annoying interface and marred with a few too many errors.
First off: The most important aspects of the game are a success. The story is great, the dialogue is amazing and highly immersive. The character-and-role-playing elements are intact and mostly well-designed. The combat is something fresh and, while unpolished and sometimes difficult, it serves mostly as a successful entity in the game. The length is there if you want it to be there with a huge slew of side-quests and things to find. The game has many multiple endings options and gives new weight to the word "choice".
And now, the wrongs: Mass Effect fails in the interface department. There are some huge problems with the equipment screen, which will take many players something like hours to fully figure out, and still it proves difficult and tedious to utilize. The combat, while fun, can be unfailingly annoying at certain points, as your team AI pretty much sucks 80% of the time. Truthfully, a KOTOR-like system would've been very welcome in the game (at least in my eyes). The vehicle sections are too plentiful, considering how silly and mediocre the vehicle-combat and driving is. The graphics, while excellent, tend to hitch often, and the framerate is inconsistent. The story itself is very short for an RPG, and one could beat this game in under 10 hours if they did nothing but the story.
The reason this last point is so miserable is that many of the sidequests, barring the ones on the Citadel (the main station in the game) are repetitive and extremely similar (unlike the quests in KOTOR and KOTOR II). Almost every uncharted planet looks like the same build with a different skin, and the interiors are mostly COMPLETELY similar. Still, there are some very interesting quests on the side which earn their merit points, besides the repetitive environments, but overall, the game turns rather bland in the side-quest department.
Despite the many and readily evident flaws the game boasts, Mass Effect is indeed a great success. The environments, the atmosphere, the innovative and fresh (as well as superbly acted) dialogue, and the supreme differences likely to be exhibited on multiple playthroughs all help this game to shine rather than to fizzle out.
So, while it is obviously marred by a few too many errors, Mass Effect is an RPG that is certainly worth playing through countless times, and therefore worth owning. If one is able to look past its obvious shortcomings, there are several great times to be had here--so don't overlook it!
And here's to hoping for the future: Bioware--fix your errors please!