Whilst Ive never used one I've seen many good reports about them ( and a few not quite so good ) .
On the down side they're not cheap by any means , as you can see on the link provided below.
Another kinda negative thing is that even though they're controlled by a built-in thermostat there are many reports of people suggesting that they can still get too hot for snakes to rest on or even lean against .
There are some guards available on eBay for an extra £25 or something but some people make their own protective guard to fit over the Microclimate AHS heater .
http://www.reptiles.swelluk.com/rept...rs-175433.html