I've just finished Mass Effect so here's my review. The writing is fantastic, really worthy of those who wrote Dragon Age Origins. The choices you make are almost all universe-changing (and since it's a trilogy, they affect the sequels too). Most of the game is just driving around in a vehicles whose physics and horrible controls make you wish you were playing Big Rigs (yes, I went there) but when you're actually on foot, the game becomes a pretty dang good FPS with a pretty simple and minimalistic skill & level system. At first when you level up, you gain four points to put in on skills but as you get higher level, you gain less. I finished the game at lvl 49 and by that point I was only getting 1 skill point per level, though by that point all the skills I needed were maxed so I was just wasting them away. The difficulty is pretty realistic, a rocket in the face can kill you in one shot so you have to do some dodging around or cover behind obstacles and shoot when you have a break. If you do good, stores will give you discounts, you'll have new dialog choices. But if you do bad... stores will give you discounts, you'll have new dialog choices. Just... diametrically different dialog choices (SPOILERS: At the very end when talking to the final boss, if you're really good you can convince him to change his mind about the whole thing, while if you're very bad you can straight up convince him to kill himself).

Overall I'd say that without the storyline it'd be a decent game at best but with it, it's absolutely worth playing. Plus literally all fans of Mass Effect agree to say that the first one is weak compared to the sequels; that they took everything that people disliked about the first one and made 'em infinitely better (like the driving parts).

Play it, people. You can actually get both the first one & the sequel (and its DLC!) in one single bundle on Steam so it's worth it!