If you're serious about quality enclosures for your animals, don't use human heating pads. And I wouldn't use the individual sticky pads from the pet store either.
With flexwatt, you can use one long piece to heat an entire rack....so you only have one power cord to deal with (per rack) and you only have to fuss with one thermostat per flexwatt strip.
That's the other thing you need....regardless of whether you use individual UTH's....or one long piece of flexwatt....you MUST have control over the temps because any of them can get way too hot when plugged directly into the wall. You have to have a reliable thermostat to maintain optimal temps.
Top four brands of thermostats: Johnson and Ranco (less expensive on/off t-stats) and Helix and Herpestat (more expensive proportional t-stats)