Didn't really like the robot dogs in FO2 myself, they seemed like kind of a weird cold and mechanical substitute for Dogmeat's warm companionship.
Kind of undermines the great relationship with a man and his dog when the dog is actually smarter than you. I'd be willing to give them a second chance in New Vegas though.
Rex isn't supposed to be that kind of a cyberdog, but rather a dog who's brain was removed, and placed in a jar atop another dog's noggin (or robotic dog's noggin. Doesn't specify). So while he may or may not be mechanical and cold, he'll still have a personality (if that matters for you).
But I'm sure he's just one possible non-humanoid companion. I'm sure there'll be a Dogmeat alternative.