Great combination of FPS and RPG elements, blended into a beautiful world with fun combat.
You start off as a Space Soldier guy in the future, and you determine where you came from by picking in the beginning of the game, which will affect how characters talk to you in the game. Upon investigating a distress signal from a nearby planet, you discover that the top member of an elite special operations group has gone rogue, and plans to unleash evils upon the universe. So it is up to you to stop him.
It varies which I talk about first, the good or the bad, depending on which list is shorter. In this case the bad is shorter, so I will begin with that.
I do not like this method of storytelling. Bioware has been praised for perfecting the art of storytelling in games for this title, where you determine how things progress with dialogue options. However, it's like reading those old books where you can pick what happens, "if you go through the door on the left, go to page 41", and calling it a great book. Some dialogue options are nice, but the crafting of the story should be up to the game developers. Also, by creating a game with the option of picking your own dialogue choices, you run into the danger of the moments in the game where you shout aloud, "I WOULN'T SAY EITHER OF THOSE THINGS". Which seems to happen to me a lot.
Also the friendly AI is merely more targets for enemies to shoot, and the brunt of killing is left up to you.
NOW FOR GOOD STUFF.
Plenty of planets to explore. I love Metroid games where you have the option of going back to places and visiting previously inaccessible locations, and the universe of Mass Effect is full of that stuff. And if you don't like that stuff, you can skip it all! Though while exploring I did have to keep a pen and paper log of where I'd been and which planets were actually explorable, so a better interface letting you know where you had been and other things would have been nice, but I never got annoyed enough for it to really bug me.
Combat is the first thing you do in the game, which is always a good idea, and when you initially find out you have infinite ammo, you just have to think, SWEET! Combat focuses less on ammo conservation (actually zero), and more on weapon quality, overheating, and Bionic powers. It's a lot of fun, and I would constantly explore, looking for a firefight.
Graphics are good, with a kind of smooth aesthetic that is very cool. I don't know how else to describe it, it just looks cool. Voice acting is superb, given that almost every single NPC you see in the game talks, with multiple dialogue options. Soundtrack is very bleep bloopy, and sounds just like a space soundtrack should. And the replayability is very high, making you almost immediately want to start a new game once you finish. Finally the arc your character takes as the events transpire is very enveloping, since you govern most of it.
Also, I Secretly think the blue alien girls are hot. There I said it.