I'm a fan of the series; I've played through the first two games numerous times. The third one kind of soured it for me, because I highly disliked the ending. I thought it was terrible in every single way, and has killed any motivation for actually beating it twice. That being said, I love the mulit-player mode. It's the most fun I've ever had with a hoard mode, and I was pretty good at it too. I really prefered the Salarian Infiltrator, but towards the end of my time with the multi, I was able to play Gold successfully with a Human Vanguard. I didn't get a chance to play through the newer difficulties that were released, however.
As for the single player, I preferred Jennifer Hale's voice acting to Mark Meer's in the first two, but felt that Mark pulled ahead in the third game a tad. My main interest in the series is the ability to customize the protagonist. I cannot think of another game that allows you to personalize the protagonist to such an extent in relation to the story. Renegade Shepard and Paragon Shepard are different, and sometimes achieve different results. While this function is somewhat cosmetic, as you always reach the same end scenario, the details may differ. This is something Bioware is pretty good at, and a feature I enjoyed in their other games. The massive scope and production values of Mass Effect just really help to set it apart.
Bioware also has a way with creating some of the best companions in any game. The only characters on the Normandy that I actually disliked were Kaiden and Jacob. They were rather bland and boring. Jacob was just a black guy thrown in for ethnic diversity, but I didn't really mind it. It was only until the third game, where he turned into a complete coward, that I disliked him. It also bugged me when I learned that he basically just leaves Female Shepard for some random chick during the events between the second and third game.
Anyway, my favorite characters have to be Jack, Aria, Grunt, Wrex, Tali, Kasumi, Zaeed, and Thane. However, I pretty much liked em' all, even the love-to-hate Udina. The guy was just a good character. I also really liked the Illusive Man, until they ruined him in the third game. I understand what they were going for with the plot, but it just felt really forced. The comics explain his circumstances better, but I just didn't like the direction. It felt too much like a repeat of Saren.
Overall, it was a fantastic trilogy, and a technical achievement.