I have a ParkPre and had a Schwinn Ranger, both mountain bikes, and had 700C's on them for a while until i scared the crap out of myself from not having any brakes. MTN bikes with 700C's are AWESOME, you get all the benefits of 700C wheels (rolling resistance, roll longer, etc) with the geometry of a bike that's flickable and maneuverable like a BMX bike.
I personally love it, but word of advice: at least on my 2 bikes, my V-brakes and my linear cantilever brakes would NOT reach the brake surface of 700C's so I would have had to buy road brakes to mount in that little hole drilled into the top of my front fork. Didnt have the dough, so Im running my 26's with Nimbus tires and it working pretty good, but i cant wait to switch back!