I've had bad experiences with Jagers, out of the three I've owned two have malfunctioned. One died outright and cost me a couple months or coral growth before I caught it (this prompted my purchase of a Reef Angel Unit) and another stopped reading temperature correctly. I kept using the second one for a while because I was controlling them with the Reef Angel by then but without that it would have tried to slowly cook my tank.
Third one that I use in my mixing container and it is still working fine but I don't care so much what that one does.
Having said that though the only type of heater that I have had over the years that didn't fail, malfunction, or start tossing current into the tank after a year or two were the Rena Stealth heaters. I used some of those for YEARS without any problems and I still have one that I use with my QT tank that is easily 5 years old at this point as it first saw duty in my freshwater tanks back then. Now granted it hasn't seen constant use over that time but it did get 2 years of solid use and then another 3 of occasional use in my QT. Unfortunately they stopped making those heaters.
At this point I just replace my heaters every year. I use two for my setup so it costs me about $100 or so each year but with the number of SPS I have and the growth of all of them at this point it's a no brainer for me. So long story short, I'd recommend a controller and save yourself the heart ache.