After having had some time to work with VoWiFi (Voice over WiFi) on an iPhone5S running iOS8.0 on the T-Mobile network I can say that it works as advertised. No setup required, no UI issues since you just use the regular App and no new phone numbers involved. It just works. Finally!
Back in 2006 when I was involved with a startup that was mainly focused on VoIP, but did some experimenting with VoIP over WiFi, that was far from the case. The company mainly used Nokia handsets, such as the E61, to test the use-case for VoIP over WiFi. While basic functionality such as voice quality performed well, the service ended up being virtually unusable because of several issues.
The setup process and UI on the handset was so complicated that we quickly concluded that an average user would not be able to use the service even if the handset had been set up by a professional or through OTA commissioning (it did exist in 2006). Other problems included how the feature phones of 2006 handled basic WiFi setup and roaming.
In any case, the biggest changes that has led to VoWiFi since 2006 has actually not come on the handset side (which may be hard to believe), but on the back end. The software platforms now integrates the VoIP layer with the carrier infrastructure which creates a more stable platform, easier commissioning and more importantly does away with the need for a separate phone number
Now the technology has an opportunity to grow significantly and become the go-to voice offload method for carriers.