Lose yourself in a huge space opera
Mass Effect is an action role playing game, set in a universe populated by many intelligent species, of which Humanity is only one, and for once perhaps not the most noble.
Even for science fiction games, which often have impressive graphics, Mass Effect is quite stunning to look at. Every location in the game has its own feel, helped by the beautiful visual presentation. Even facial modeling, which is often a let down in gaming, is very well done. The soundtrack is suitable cinematic and dramatic, making the game hugely engrossing.
In combat terms, Mass Effect is a third person shooter, and while it's competent, it's no Gears Of War. the enemy AI can also be pretty stupid at times! Nevertheless, it's still enjoyable and satisfying. There's a lot of talking in Mass Effect, and the dialog system is excellent. You can control the direction of conversations, by choosing to be nicer, neutral or aggressive in your responses, so you can give protagonist Shepard the character you want.
Mass Effect is an RPG, meaning you start off fairly weak, and get more and more powerful as you gain experience. This may test some people's patience, and it means that combat only really gets fun once you've got a few hours gameplay behind you. The thing that will try most people's patience though is loading times. There's a lot of waiting, and in fact Mass Effect's elevators have become notorious for their slowness!
Mass Effect is an ambitious and expansive action RPG which tells a great story in a beautifully realized universe.