My TruPhone experience...
I installed TruPhone on my unbranded N95, it doesn't work on some network branded models.
It was very easy to install and set up.
The integration with the phone is seamless.
The call quality was as good as over GSM - unlike, say, using Fring and Skype - which is rubbish.
The only problem I had was stopping the N95 automagically hooking up to TruPhone whenever it was near a previously registered Wireless network. This lead to the N95 battery getting drained very quickly when I was in the office or at home. It was this problem that caused me to uninstall it. Shame really, because otherwise it was great.
Free calls to UK land-lines and cheap calls everywhere else, not surprised they're struggling to make money out of it though...