I have the Apolo BioFin, which is as close to the V12 as you can get (actually, it's the other way around) and I love them. But I can't stress personal pref and type of diving strongly enough. My buddy has Volos and loves them. We switched in the pool one day and each of us hated the other's fins.
The volos use a small flutter kick. You don't have to do the full leg kick because of the hinge. What you get is about 60% of a regular fin on the downstroke, but you get that 60% on the upstroke - as opposed to nothing on the upstroke of a regular paddle fin. A full kick cycle give you 120% rather than 100% of a normal paddle fin cycle.
With the V12, you still dn't get the power on the upstroke, but the way they channel water and push you is so much more efficient than any paddle fin (IMHO). One thing that I've heard, but not really noticed is that the vortex that comes off the v21/biofin/imprex/other split fin goes back quite a ways. This could cause some problems with stiring up the bottom even if you aren't on the bottom...maybe 2-3 ft off, finning upwards. I really like the fact that they are so pliable, they're like fish tales-no effort at all. The only down side is if you'rein a current. The natural instinct is to kick really hard to get back to the boat. If you kick as hard as you can in a split fin, they just open up and you end up with about 3 inches of effective fin and you're going to loose ground. If you slow the kick down - and this takes a concious effort not to kick like hell when you start moving backwards- you can make headway against the current.