The hardest part of the watch portion is just the testing and debugging. It takes a long time to install on the device and get a real feel for the speed of the app.
The other hard part is figuring out what feels right on the wrist, that part I'm still figuring out.
I thought about doing some sort of free version of the app with IAP, but it was very hard to find a way to do it that didn't make the app feel too restricted. I might toy with the idea in the future, are there any good examples that you like?