My wife and I started running in them, starting with walking only, then running only a bit at a time (1/2 mile run, then walk another 1 or 2 miles). Now, about 3 weeks on, we run about 2 miles straight at a time (we've never run at all before, which is supposedly ALOT harder with VFFs than if you had started out with cushioned running shoes).
At first, we had pains in places we'd not had before being on elliptical machines or my wife doing aerobics, especially in the shins and forefoot and midfoot. I got some slight blistering on the big toe the first week as well, but it's all gone away over the subsequent weeks.
The verdict? It's about as fun as running as a kid barefoot on a summer's day. We actually look forward to doing 5k in them. Then, we go trail running, which I'm sure will be a blast. It seems to feel alot better underfoot on soft dirt than paved surfaces, anyway.