All in all this is a very good satnav unit. Having previously owned a Garmin Nuvi 660, and having returned it due to the appalling TMC reception, the access to TomTom Plus traffic services is a definite plus. The TomTom also has much more customisability than the nuvi e.g. you can define an itinerary, or set a route that avoids a particular road.
However, there are other things that I miss. The display of traffic info when browsing the map could be simplified. It is often hard to read, and clicking on the traffic POIs can be a challenge while driving. However the unit does highlight delays relevant to the route, although the calculation of delay times appears somewhat exaggerated, and should be taken with a degree of caution. In fact, the TomTom's calculation of journey times in general appears to be on the high side.
The other thing I miss is the text-to-speech facility that allows the nuvi to tell you which street or motorway you're driving on. And the nuvi also highlights mobile speed camera zones, while the TomTom only displays fixed camera positions. Talking of display, there doesn't seem to be a way to get the TomTom to switch automatically from day to night-time display. There is on the nuvi.
Accessing traffic data whilst en-route also ties up the bluetooth on your mobile, and my phone is unable to connect to my hands-free device at the same time. As the One XL doesn't support hands-free calling, this is a big minus for anyone who wants to use their mobile phone legally on the road, and access Plus services.
Finally the windscreen mount, as mentioned by others, is prone to falling off on a regular basis. Unlike the nuvi's.
All in all, a perfectly functional device, albeit lacking some useful features, and a little overpriced for what it is - you're essentially paying a premium of around 70% on a TomTom One, for the privilege of a wider screen.