You'd probably know Bioware by Star Wars: The Knights Of The Old Republic or D&D-based Buldar's Gate series. The developer has been making very good role-playing games based on the world settings created by somebody else; which is not such a bad thing, as they are very well made (KOTOR is considered very highly by many, and Buldar's Gate II is one of my all-time favorite RPSs).
Mass Effect breaks away from this tradition. This time, they have created the entire universe from the ground up. What's unique about the world of this science fiction RPG is, unlike many other sci-fi settings like Star Trek and many others, humans are not the center of the galactic federation of interstellar civilizations. Humans are newcomers, only recently acquired the technology to go beyond its own solar system; many other races treat humans with mild tolerance, although some regard them with outright prejudice and hostility (such as elitist Turians- they clearly think mankind is not worthy to be joining the party). The background of each race and how they came to travel back and force- using Mass Relay, basically a conduit to let you teleport from one solar system to another, left by ancient, long-gone civilization, is very detailed very well-thought out.
You play as Commander Shepard, the first human candidate for the elite peace-keeping force called Spectre, which apparently has the authority to exercise extra-judicial deadly force to enforce law and keep things in order. Having a member among Spectre is a big step for the mankind. You can make your Commander male or female, and edit the way s/he looks. I just picked the default (which is male), he just looks cool enough.
Things immediately go wrong as Saren, a Turian Spectre, gets hold of an artifact left by some long-gone civilization. He, being a Spectre, clearly thinks that if he can be above the law, he can above many other things (he's probably never watched Spiderman). Before long, you are put on a mission to stop Saren from his nefarious plan to destroy all the civilized life form.
(The story doesn't miss the political power play; at first they want you to spearhead this mission, because even if you fail, no other race will take the blame because you are "simply a human" after all) You'd get to scout buddies, human and non-human, to tag along with your hunt for Saren and uncover his true intentions. Each of these non-player characters has personal history, likes and dislikes. (Taking a certain NPC to a certain mission triggers a special dialogue/confrontation)
As long as you issue relevant orders (it would be a very good idea to tell them first to "take cover"), they behave fairly intelligently and help you beat your enemies. They do have some path finding issues and, given no order, just want to storm in and become a good target in the open.
Saren is one bad mofo and will keep your hunt quite worthwhile. Many important story sequences play out as cut-scenes, and they are mind-blowingly good. Especially after one big confrontation toward the ending, things will get really intense; you'd enjoy it more than most of the sci-fi movie made out there. Dialogues are pretty well-written, won't be called literary masterpiece but for the ones gracing a videogame, they are quite impressive. Aside from a major gripe (discussed below), this is a spectacular achievement and is a great start of the new saga (there will be a sequel).
The story itself and its conclusion is very tense and very satisfactory, there are a few big surprises along the way and it will keep you going- and especially the last 5 hours or so will be quite memorable.
It's not without is faults, though. One word; side-missions. They suck. You get about a dozen star system you can visit to check up on some research base that lost contact, uncover something or go rescue some squat that put out a distress signal. All of these worlds are made of one map, and the location you go to for your mission (research facility, mine, etc)- many of them are made of the same design. I don't mean similar. They are exact same map, they only put crates and stuff for variety. You try these missions to garner experience and level up your characters and find better weapons, but they get tedious real fast. (You could probably beat the game without playing any of the side-missions; which may give you a better memory of this game.) All in all, side-missions feel like an afterthought, like they put it because they have to be there. I hope they will make them more varied and interesting in the sequel (at least, they should take place in terrains and maps with slightly more variety).
Also, inventory system is a nightmare. It's supposed to give you many upgrade options to your weapons and such, but doing so is kind of a mess. I hear PC-version's got a bit better on this front though.
All in all, a very solid role-playing game (as long as you stay away from side-missions) that can be recommended to any sci-fi fans and RPG fans, especially RPS fans who like science fiction.
Good: Great story, excellent cut-scenes. Very detailed and interesting world setting. Fun combat, once you get the hang of it (though later it could be a bit repetitious). Interesting characters. You get to hoop up with opposite sex members of your crew.
Bad: Horrible inventory system. Side-missions. Minor path finding problem with your buddies. Did I mention side-missions?