For what it's worth, I think the TomTom is great for the type of journey where you just want to navigate to an address by postcode taking the fastest / shortest route. If this is all you want to do get one. It's a no-brainer, IMHO.
If you want to choose your own routes, perhaps taking in some interesting roads, and plan routes in advance for using later, the Tom Tom is not for you, IMHO.
Yes, it comes with software you can use on a PC for planning routes "offline" but this just brings up a "tom tom emulation" on the screen so you get to plan routes just as awkwardly as on the tom tom itself but with the additional hassle of having to transfer the routes from the PC to the Tom Tom.
In contrast, I used to use CoPilot Live on a Pocket PC and this came with a really good mapping application you could run on a PC to enter routes. Nice, big, detailed maps of the whole of Europe to plan routes on, and then just copy them to a memory card and chuck it into the PocketPC.
Yes, the PocketPC solution made it very clumsy on the move as it's an unstable platform that has a habit of suddenly denying any knowledge of the bluetooth GPS, running out of memory, etc. It relies on more cables than the Tom Tom and is generally a pain. The CoPilot software is not desperately stable either, and, in particular I found a roundabout near Spalding that makes it throw it's toys out of the pram in a big way every time I go past there.
The convenience of the Tom Tom on the road with the offline planning capabilities of Co Pilot would be fantastic. :-/
I bought a Tom Tom before driving down to Italy last year and really missed some of the features of CoPilot.
Kevin