I'm going to be getting a pair of VFFs this week. I've been doing a mix of running, stairclimbs, P90x and Crossfit WODs for about 6-8 months, but I developed an unbearable case of shinsplints over the last month or so. It started when I added stairclimbs with a 75lb vest, but only hurt bad when I ran....sometimes I'd get 4-5 miles, sometimes I'd get a few hundred meters.
I plan on using them in some Crossfit WODs with some 400 meter runs to start out. I'll start adding some distance running after about a month or so.
I've heard they (VFF) require some dedication to re-learning how to run properly....so I'd take that as a pretty serious consideration before I give them a shot.