Flown it (as in drove it not rode it):
ATR42-500 and ATR72-500: Lovely to fly hand flew most of the time (was for a magazine article) really very forgiving and a lot of fun to fly. The -500s have had a bit stiffening work done to the upper fuselage and have six blade props all of which makes for a reduction in vibration and noise. Only downsides - I'd like to see some nose wheel authority via the pedals (just the tiller on the ATRs) and you want to be careful you fly the nose wheel on on landing.
Dash 8-Q400: Again lot's of fun to fly bags of grunt and it goes like the provibial waste product off a exacvation implement. Had a few probs with realiability early on but I gather that's been pretty sorted now.
I've been told (by them that know) that the seat mile costs on the the 70 seaters (Q400/ATR72) are pretty much the same as the 737-3,4,7,800 so for routes where you can't fill a larger a/c they're just the job.
While fuel pricing stays where it is (Jet fuel is about 400% it's May 2001 price) the superior fuel economy of the turboprops will continue to make them attractive to airlines (no matter what the punters think!). The proff of the pudding...both ATR and Bombardier are shifting them at rates much much higher (almost twice) their most optmistic forecasts of just a few years ago.
Interestingly although the Saab 2000 is out of production they are rarer than hen's teeth on the second hand market (even with SWISS phasing out their fleet (they were the largest operator). Meanwhile, in the US there are more S340s (out of production for even longer) in airline service now than there were a year ago.