I didn't buy it yet. I loved ME1. Actually I'm a RPG enthusiast. I played RoleMaster for many years, which makes (A)D&D look simple. So actually NWN(2) is probably the most complex RPG ruleset ever implemented in a computer RPG. So from this point of view, ME1 is a RPG light at it's best.
So, why did I like ME actually more than NWN 1 and 2? Actually even more than KotOR (especially if you subtract the Jedi bonus).
a) The superb storytelling with an awesome setting, interesting characters, immersive gameplay, wonderful dialogues, great atmosphere, and emotionally moving moments and decisions are a core of a storytelling kind of RPG, which puts the focus on the story instead of the accurate world simulation.

Combat in Pen&Paper RPGs is generally round-based. Why? Because the DM usually needs to ask all players what they want to do. That can't be organized in real-time by a group of human players describing their actions. Sure, this also makes combat more tactical like in DA:O. But in ME there is one player. The computer can manage many simultaneous actions. Also action TPS combat suits the ME series even better than traditional turn based combat. Still there is a pause/interrupt system, so the player can give tactical orders without hassle.
c) The cinematic presentation is unparalleled by other games so far. The game feels like an interactive movie while providing an active straight-forward gameplay.
Actually I hoped ME2 would also take the RPG part to a higher level. Currently it seems Bioware maybe streamlined the technical aspect of the RPG part too much. But then again it seems the storytelling aspect has been even improved, even with the story of the middle part facing the challenges similar to the Empire strikes Back.
Why didn't I buy it yet? I am unsure if I will like ME2 as much as ME. Reading, that some major decisions of ME seem to have had a rather insignificant impact on ME2 made me stall. I know, you can't design two completely different story-lines e.g. depending on what happened to the council. Well, I'll have to see how the game turned out to be by myself.
The other reason was time. I will have more time in February... Just debating, if I finish my second playthrough in DAO before... Was a long post, but I felt like sharing my thoughts.