Mass Effect is a great cross between and RPG and a third-person shooter.
You start out by making your very own Commander Shepard. Like many RPG's, you can choose the appearance, sex, class, and even your character's history. Once your character is created, you are thrust into the action. The battles are in third-person, and you control Shepard. Your two allies will assist you and even talk to you as the game goes on. If you try to play this game like purely like a shooter, however, you will probably end up dying a lot. Instead of old-school "magic", many characters have abilities called Biotics and Tech Attacks to throw enemies, hack AI units, and a number of other effects to help you out in battle.
After the first couple of battles, the story begins to unravel. As a Commander in the Alliance Fleet, and the very first human "Spectre" agent, you must track down and capture Saren, a rogue alien who has sinister plans for the galaxy. It seems simple at first, but the story really shows off its depth as you progress further in the game.
Technically, there are really only five main "dungeons", and the main game could probably be completed within 10 hours for those who like speed. But a hefty number of interesting and action-packed side quests will keep most players going for a long time. Overall, it took me 28 hours to complete the game, and I didn't accomplish many of the side quests.
So here's the Pros and Cons.
The Good: The characters and dialogue hooked me from the very start. Each character has their own back story, personality, and, of course, fighing style. Some characters you will like, and some will make you cringe every time they talk (Liara), but most of them have excellent voice acting and well-written lines.
The story seems fairly simple with a few twists and turns thrown in. But the intense character confrontations REALLY drive the story a lot, especially when your dialogue choices affect the outcome of a scene.
But the thing that really stands out to me about this game is the combat. There are just so many ways to take out your opponents, from shooting the repeatedly to using your biotic powers to throw them off a ledge (my personal favorite). You will often find yourself thinking of strategies in your head along the lines of: "Okay those two are dead...oh those three are in a group I'll use life to get them all...hmm better toss a grenade to take out those guys behind the shields..." The list goes on. After each battle, you will be trying to find more battles to get more equipment, to level up, or just to kick ass. It really requires more strategy than reflexes, since you can pause the action at any time.
The Bad: Not much in this category, but still a few things worth noting. As the GS review says, the vehicle combat is fairly clunky, and all the vehicle combat sequences really aren't much different. Also, the side quests, while very fun at first, eventually get fairly repetitive. Basically its like: Go to planet-->Find fortress-->Take all bad guys out-->collect loot. Also, for people who like to grind levels, it is important to mention that there is no free combat. All fighting is done in the main quest or side quests.
Overall, Mass Effect is a stunning game. It's action-packed, well-scripted, and VERY addictive. You will probably see it like a movie at some points, due to some intense cut scenes. I'd recommend this game to anyone who likes RPG's, shooters, or sci-fi series.