I love my Z with the camera mod - but now the phone battery's starting to fade, the camera craps out if the phone battery is at less than 70%, and sometimes the phone crashes too. I've dismantled a previous camera (met a sticky end when I rammed it into a tree), and there's nowhere easy to stick a big power smoothing cap.
Replacing the battery in the phone looks non-trivial on youtube, but I'll probably bother, as the combination is really nice. Seamless bolt-up to google photos if you don't mind google having your photos.
The speaker mod (with its own battery) is great, runs for many hours, plays audiobooks or radio while I'm working. You really know when an SMS has come in, though.
I've also got the development mods, but have done nothing but fetch the toolkit and run the demos. Anything that I want to build, would probably be better connected over USB or Bluetooth. An oscilloscope back would make me very, very happy. Sort of bolt a Red Pitaya or whatever on, but use the fast camera data lanes to dump a screen buffer onto the phone.
It's a shame there's not a sandwich battery mod, so I could run the camera forever. An extra 4mm thickness would be a massive capacity boost.
Anyway, mods = gods, etc. And the phone, without mods, isn't too shabby.