This, of course is an age-old debate. It appears that on shorter, lower-altitude routes turboprops, especially latest one such as the Q400 are, in fact, a better bet.
Our airline currently operates CRJ200s and 700s. It's been calculated that on routes of 350nm or less, the Q400 will land perhaps 10 minutes behind the CRJ200, but will have burned some 1500lb less, while carrying more people.
This is one example. Here's another one:
For my previous job, I flew a Piper Meridian. We used to be able to block 260 at 260 at 260, meaning 260KTAS at FL260 at 260pph. This meant that on routes of less than 500nm, we were only marginally slower than, say, a CE500/501, but at less than a quarter of the operating cost.
You can do the math from there.