I have a Ranco, a couple of Helix and a Herpstat. All of them work perfectly.

If you are using lights to heat the cage, I'd suggest the proportional (helix, herpstat) thermostats as your lights will last longer. It's not as hard on them to dim as it is to shut on and off.

If you're using heat tape/flexwatt/etc, there is nothing wrong with the Ranco. I am using mine to regulate all of my ball pythons in two racks and it works perfectly. I have had no problems with it at all and it keeps the temps pretty rock solid (within a degree of the set temp) and has been running like this for a year.

I would advise against the zoo-med thermostats, only because in all the failures I've read about they tend to fail ON, which can be a big problem if you don't notice it soon enough. Sure, they will work great up 'til that point.....