I have also had good luck with the Visitherms in that they have never cooked fish. I do find that they need calibration more so than some other brands, but that is a small price IMO to avoid cooked fish. I had two hydor in tank heaters, and had one cook fish, the other is still in operation and is about two years old (?) so it could have been a design flaw. I have also had cooked fish with Rena's, Ebo's and the dreaded Tetra heaters. But IMO if you are running a canister, it is worth the extra $ to go with the Hydor inlines, they are rock solid, and the heater isn't in the tank. I would love to switch all my tanks over to them eventually. Hope that helps.
I'll have to be the voice of dissent. I initially used Hydor's in all my tanks. However, after 1-2 years, the temp sensors failed in all of them. Most of them became "stuck on" with one of them failing altogether (causing my reef tank to drop to 68F over night).
I've since replaced them with Visitherms and Stealth heaters.