The Mass Effect trilogy is one amazing ride from start to finish. It's like a novel, movie and game all rolled into one.
I would argue that it rivals sci-fi greats like "Star Wars". Nothing could have prepared me for the epic ride I was about to take through that amazing universe. I had never played a game that made me feel like I was part of it. The sense of awe and wonder as I explored new planets, made choices that changed the storyline was unparalleled. I have never, before or since, played a video game that put me on such an emotional roller coaster. I was so invested in the characters, so invested in the plot. A game never kindled that feeling in me before. It will always hold a special place with me.
I remember reading that Star Wars was one of the influences for Mass Effect when they wrote it. The effort and level of detail they put into the lore of the game made me want to live in it, LOL. I think the last time I felt that kind of attachment to a universe was Star Wars and it wasn't even on the same level. It was the only video game franchise where I read the novels and the comics written based in the universe. Hell, I'm even tempted to write stories that simply takes place in the universe (but it'd probably be labelled as fan-fic anyway). I haven't felt that desire since I was in high school. The trilogy has a special place in my heart and I have to say I'm relieved they're going to another galaxy with the next installment :)
And it's not just the planets or the story, either. I don't know about you, but those games make me really intrigued by the technology present in them, and what living in that universe would be like. Plus, Garrus is kinda the coolest..
I'm not familiar with the co-op classes, but that sounds awesome. However, I personally prefer playing as an Infiltrator (and a Renegade one at that). OK, he's definitely the coolest. Or, at least tied with Mordin.