I would definitely recommend Thermalution. A few of us guys are wearing them around here and swear by them. You can't feel the suit underneath your wetsuit or drysuit, the low temperature setting generates a good deal of warmth for 2.5 hours, and the medium setting is TOASTY warm and will really keep those back muscles loose during a sesh.
I wore the Quicksilver Cypher Vest and wasn't very impressed. The battery was in a horrible location for kiteboarders, and I just didn't get the level of warmth I expected from a battery heated rash guard vest. The Thermalution is a different story and makes a big difference even on Low. The medium setting is by far the most I've ever really wanted; the hottest setting is a little too hot, especially with the harness pressing the warmth into your back. The battery usually lasts me a little over 2 hours, riding constantly on low, switching to medium every now and then if I wind up having to body drag a long way to my board and end up getting flushed and chilled.
TWO 7.4V Lithium Polymer batteries are smaller than an Iphone, and sit much higher and rest just above the top of your harness. They take about 2 hours to recharge from the wall charger. I have not tried the car charger.
But yeah, I'll definitely give the thing a big thumbs up
! They are sweet! Mine has made my winter sessions MUCH more enjoyable this year!