Great storytelling and enjoyable characters make this game excellent. Bland side-quest can get tedious.
The best part of Mass Effect is its story. As Shepard you'll meet up with many different characters and situations and they way you choose to react to these situations affects the entire galaxy. By the time the game ends, its very easy to feel connected to your personal version of Commander Shepard. The ending will differ based on your decisions and because of this, the game promotes multiple play-throughs.
The main quest will take about 24 hours of play time complete but that can be more than doubled by completing the games side-quests. Sadly most of these assignments are extremely boring. The majority involve driving around in the your tank-truck, the Mako, on a nearly desolate planet. While many of these side missions can add to the story, they do so by a text box popping up and explaining what happened. The nice part is that you never are forced to do a single side-quest so if you decide you don't like them then you can just skip them.
There are a few other odd items that keep Mass Effect from being perfect. There is a lot of texture pop-in and while its better than loading screen, it still breaks the immersion. Also, I find the driving sections quite boring, and its not helped by the Mako controlling poorly and sometime not shooting where you aim. Many players complain about the elevators in the game, but they are not a big issue. You can use the in game "fast-travel" option to almost completely avoid them.
All included, Mass Effect's story, presentation, and gameplay make up for its few faults. With all the choices a player can make its easy to finish one game, just to want to immediately start another. I've completed the game many times and still enjoy playing. Whether you'll play through multiple times and get the value out of it I did, or for just one quick run-through, Mass Effect is an incredible experience that really shouldn't be missed.