The first of the Mass Effect trilogy sets an innovative and entertaining pace that solidifies it as a must-play game.
Mass Effect is not originally a PC game, and the PC version is a direct and not particularly well-done console port. The graphics support is meager for its time period and there are plenty of control anomalies that will initially bother PC gamers. These will all soon be forgiven, trust me. Some strange design choices are puzzling: long boring elevator rides, a wheeled all-terrain vehicle that is essentially indestructible and can scale nearly sheer cliff faces, an inventory system which is nothing short of befuddling, environments that recur over and over, and some other very minor issues pop up from time to time. There, that's out of the way.
Mass Effect tells the best story of any game in the last decade. Not only is the story itself great, but BioWare uses the game to tell it in a marvelous way. It takes no time at all to buy into it and feel the full weight of the impossible odds that you and the intrepidatious Commander Shepard will be facing. It has the ability to completely suck you into the story, and keep you guessing and wanting to know more about what's going on around you.
The interaction with the crew is perhaps one of the most rewarding aspects of the game. Each crew member has a deep back story and litany of personal problems, and talking with them is a very worthwhile venture. In fact, it says something about the writers' abilities that you actually feels as though you get to know these people. You become actual friends with them as you share the adventures facing your team - or enemies, as the case may be.
The combat in Mass Effect feels authentic and is great fun. Combat is essentially the only RPG-esque element of the game; you can customize Shepard's abilities in several different areas of expertise, any of which will make a distinct difference in how you and Shepard will go about saving the galaxy. There's also a karma system (Paragon/Renegade), which allows you to choose what kind of person the Shepard that you are controlling is going to be. These RPG elements are the primary reason you'll want to play through Mass Effect several times: once as the gun-specialist good guy and another as the biotic warrior jerk, then another as a Lady Shepard sniper, etc. The fun never seems to end.
There's nothing else I can say about Mass Effect other than you must absolutely play this game. Remember to develop several good characters to their conclusion, because you will import them into the 2nd and 3rd Mass Effect games to form one continuous storyline of your own devising. Good luck out there.