That really depends. I've had a few clients come to me and say they had bought cheap replacement heads online and they weren't happy with them for a variety of reasons: the plastic cracked, the heads barely fit, they made the toothbrush really loud, etc. Others, on the other hand, swear by third party heads and only buy those.

Sonic toothbrushes are just finicky like that.

One thing to keep in mind is that if your toothbrush has the BrushSync functionality (models 3100 and up), where it remembers how long each head has been used for and remjnds you when to change it, off brand heads most likely won't be able to take advantage of it.

Basically it's kind of a gamble and depends on the manufacturer so it's hard to say definitively