I think it looks really interesting, especially the idea of not having to carry "yet another gadget". It supports downloading traffic and weather info over a GPRS connection. But as Pycage mentions, the prices is a little high - 200 Euro for just the Nav upgrade. And it's also Europe only at the moment, not so useful in the US or when traveling to New Zealand.
Maemo Mapper by comparison is free, and hackable. It just doesn't come preloaded with maps or quite as many features.