my wife's new bike has hayes hydro discs, i test rode it while she was at work
and found they can actually skid on dry pavement under my 210 lb butt!! i was really amused, i have tektro v's on my bike and can't get it to skid ever, at all, oh wait it can on a steep downhill of soft dirt or gravel.
however, i still don't think i need discs, my v's have never failed me, i've never not been able to ride something because my v's weren't good enough. and, i already have them, if i were building a bike from scratch i might opt for disc, but since my v's are fine now i'll just keep them.
though once my v's start to wear down the wear indicator line on my rims i might upgrade to discs rather than buy new rims, because my fork, frame, and wheels are all disc ready. but the bike is new so thats hundreds or thousands of miles away.
oh and about terrain, my v's don't stop quite as good in heavy rain, but i just clamp harder and they're fine. and when they get muddy i take a kleenex to them (or a leaf if in the woods) and they're fine.