I'm using this one from Summit:

http://www.summitracing.com/parts/HDA-3653/

I've had it in for two summers and have had no problems with it. It's adjustable and has a probe, like the one you're looking at. It just took a little playing around when I first installed it to dial it in so that it comes on when I want it to, and I haven't had to touch it since then. It's been very consistent, coming on at the same temperature (according to my temp gauge) every time. Even though this one has a built-in relay, I'm using a second relay for the power to the fan, figuring that the thermostat will last longer if I'm not running high amps through it.