Eh, not sure if you want to hear an opinion from the other side of the fence, but...
I'd say Resistance 2 is probably closer to the most overrated PS3 game around (IGN score, anyway). I feel like the reviewers at the time were caught in the fanboy wars, and were pressured to give R2 and Gears of War 2 the same scores in order to be "neutral." The fact that IGN didn't include it in the top 25 PS3 games doesn't mean anything, though, those lists are crap (Gears 2 didn't make the 360 list, either, while Halo 3: ODST did).
I find the Resistance series to be pretty mediocre in general, it's the most generic FPS I've played in a while. I felt like it had introduced nothing new from the FPS I've already played in the 90's. Its only saving grace is in its split screen modes (campaign, online), primarily because so few console games bother with local nowadays.
The fact that Resistance 2's sales are so poor compared to the first can only mean a few things: poor reception from either game, lots of bundled sales from the first, or FPS saturation. I think it's a mix of all three.
