I just bought a SIP WIFI phone. I got myself the UTStarcom F3000.
With it all setup, I was having lunch today in a local café, someone called my home/office phone number, the Sipura 3K at home routed the call to my virtual PBX, which rang the WIFI Phone, because someone upstairs from the café had an open network which the phone had already used to register.
I have only had it for two days. I am amazed how many open hotspots there are around central Brixton!!! Central London is even more crowded with them. Unfortunately, the phone is not always able to connect to open networks, maybe they are locked down using mac addresses or something .....
The concept is wonderful, this implementation is not.
The build quality is OK.
The battery life is pathetic.
The interface is unpleasant to say the least.
The ringtones are obnoxious.
It has a built-in webserver to configure it (cool huh?), but there is no way to add people to the address book except via the keypad (AAARRRGGGHHH).
It feels like a phone from 10 years ago.
I do not imagine WIFI phones will ever get the level of connectivity that we have become used to from the GSM networks, but being based on open protocols means that there will be a much higher rate of service innovation than that provided by the dead hand of the big GSM players.
The ideal would be to have both systems on one device.
UPDATE: I have had the device long enough to say that the battery life is not so bad after all. As long as you do not leave it on, trying to find, but not finding an access point, it will happily standby for a couple of days.