I have installed them in the past. Most homeowners enjoyed the constant temperature/lack of temp. swings. The only real problem I encountered were with the solid state relays on the early models. The relay would literally pop open apparently from overheating. They are very specific as to where this relay gets mounted to prevent this problem. I stopped using them when manufacturers came up with their own version of the same principle. Lennox has "Even Heaters", York or Carrier has "Smart Heaters", etc. I would look at the OEM version first if available for the system you are considering.
There is nothing more frightful than ignorance in action....Mark Twain