Pretty much everything about them. Fit & feel, machining, weight (my Titianium ProVari 3 weighs amazingly little), features like initial boost of power (why oh why does nobody else do this in wattage mode???), other features, user interface.
They're just all-around exceedingly well-made, and extremely good performers.
The company is also top-notch for support and just all-around personal qualities of each and every person I've dealt with over the years. And since my wife & I have been buying ProVape stuff for almost 7 years, we've had occasion to interact with them on several occasions, and they've treated us perfectly.
We've had 2 ProVapes go bad in some way. Both were fixed at no cost - and one was over 3 years old. They've no-cost shipped spare parts for my Kabuki - OK, the stamp cost more than the parts, but they still did it without blinking. Other interactions - questions via email and such - have been quickly answered in a most professional manner.
I have never ever though twice about buying a ProVape device every time they've announced a new one. Until the Procyon, as I mentioned above. And if they announce a 75-watt 0.15 Ohm Procyon, I will order it. But I hope they do that to a Radius because I'm not so into tube mods any more.
Damn, I sound like a fanboy. But I'm not. It's just that the ProVape devices and people have been as close to perfect as any company I have ever dealt with.