I made the client for domoticz, I chose it after some tests and it fit my needs.
About the protocol, I use ZWave, not open sourced, but there are opensource controllers.
I chose ZWave because of the wide ecosystem of sensors and actors and because I needed a wireless meshed network.
I like Fibaro's design and products in general but I tried several brands for light bulbs.
Regarding the price tag, in Europe you can find various ZWave sensors around 30€ (seems to be the minimum price so it should cover the ZWave licence)
I'm interested in your findings as I would like to have a more open domotic universe at home.
If you use domoticz, I would love some feedback to improve the UX.
(I'm working on adding local caching and customization but it's not ready yet)