I've just 'upgraded' to TT5 and it's being used with an IPaq 5550.
It does seem easier to use and besides an issue with the 'bluetooth' receiver easy to set up. FWIW if you use a non TomTom BT receiver, you would need to set it up as a 'other NMEA cable device'!!! details I eventually found on TomToms website support.
It also has the address by post code, and the option traffic and weather via a mobile phone.
Also comes with maps of the whole of western europe.
edit to add; I believe it doesn't work with ipaq hz47xx and some of the other PDA's.........................yet!