Mass Effect 2 is one of the great games of the past decade. The combat is fast, exciting and engaging. The characters were (by-and-large) believable and the dialogue was very natural and well-acted by videogame standards - both were miles ahead of Dragon Age in my view. The plot didn't have the same wow-factor and open-ended feel of the original, but I found it pressed my emotional buttons more effectively and one of my constant frustrations with modern RPGs is a lack of focus on the main plot.
Having said all that, I can see why a certain type of gamer would be disappointed with the direction they went in with ME2. Linearity grates on some people, particularly RPG fans, who want freedom to explore. My own opinion is that this is what Bethesda games are all about and Bioware tried their hand in the original Mass Effect - I found Oblivion and the exploration sections in ME tedious and detracted from the main story. I preferred the smaller-scale, more "rail-like" experience of ME2's sidequests.
Personally, I would have preferred a more complex and optimisable character-levelling mechanic, but that's a small complaint. Perhaps it just needs to be made clearer that Mass Effect is very much on the action end of the RPG spectrum. It's big on story, big on character, big on action, but not big on roleplaying.