We use both TomTom GO (all-in-one) and TomTom Navigator (same product based on a PDA) in our company, I have an iPAQ 3970 PDA which has had TomTom 2, 3 and now 5 installed. Living near Oxford and working in Rotterdam, I've always had the full maps of Europe installed.
I was getting ready to buy the TomTom GO 700, but decided to try the TomTom Navigator version 5 upgrade for Euro 59.
The good news is that TomTom Navigator 5 software has worked without fault for months, no lock-ups, no need to reboot, the user interface and maps are identical to those used in the GO. Basically it is rock solid on my iPAQ and I'm very pleased.
Our GO 700 (full Europe maps) users see exactly the same user interface, the same maps, but the GO is faster at searching, and probably has even better software stability - which I'll explain now:
TomTom Navigator is based on a Windows driven PDA - like my iPAQ - and in addition to TomTom, can also run an agenda, an address book, e-mail client and any other application you care to buy. If you want to, Windows will let all this stuff run at the same time as TomTom - sometimes it is nice to just go to the address book and have TomTom navigate you directly to that address - sometimes the other stuff gets in the way and slows the PDA down.
TomTom GO is a dedicated system, based on similar computer hardware to the iPAQ, but with a version of the Linux operating system dedicated to running TomTom. Consequently it's only running one application, in responsiveness and searching it's faster, and there is no chance for undesired interactions between programs - it can only run TomTom
Suggestion: If I was buying now and wanted door-to-door mapping for all of Europe, I would buy a TomTom GO 700
If I wanted door-to-door mapping of the UK, then the GO 300 (UK maps only) is a good choice and I'd buy additional country maps when I needed them.
The GO 500 (UK + major roads of Europe) could be OK, but the maps of Europe only take you from city to city - they don't have the fine detail to take you to, for example the Delftse Hout camp site just outside Delft in Holland.
Do I use the PDA functions on my iPAQ? - No, it now only runs TomTom - so I'd save the money and buy the GO
Warning: Dependng where you live, you may not want to advertise that you have a GPS navigation system in the car - the screen mount is a give away - just flip the clip and remove it if you park anywhere doubtful.
Towing with GPS: When you set up a destination, select the route that prefers Motorways. None of the navigation systems currently understand the special requirements of caravaning, or large trucks.