I've used one of these extensively on the Isle of Man, in the UK and in France (obviously, I have the Europe edition but the only difference is the additional maps). I also have a great deal of experience with Tom Tom.
As the Nuvi 250 not a top-of-the-range unit, there are some shortcomings - and overall I would prefer the Tom Tom interface.
But it is extremely neat, lightweight and easy to set up (which means you're not tempted to leave it on display in the car when you park up for a few minutes and nip into a shop). Actually using the unit for the main purpose (getting to an address) could hardly be easier. It's never locked-up or crashed despite some multi-hour sessions and varying temperatures. The windscreen mounting is excellent: the best I've seen and very easy in any car. I have to admit that I bought it for my wife, who prefers such things to be extremely straightforward (any why not?): she is very happy to use it.
The biggest weaknesses are the voice (very robotic, and can't say "right" correctly which is irritating)(later edit: this has now been fixed by a download from the Garmin web site) and the 3D map display (no indication of movement, and confusing where there are many streets tight up against each other). Also, the location of junctions takes some getting used to: if you wait until "zero feet" before turning off the motorway you'll have missed the junction by a long way. I think that this is partly the mapping, and partly that the countdown of distance to go is a little slow. Moving at 30 mph, a left turn will be indicated ahead but the actual turn takes place whilst the unit is still indicating 50 feet to go. If you stop at the junction it catches up a few seconds later. Tom Tom on my PDA (admittedly a high-power one) seems to stay in sync better with the position of the vehicle.
It does hold satellite lock very well (as you'd expect with this chipset), and generally starts up quickly.
Sometimes the route chosen seems a little odd: in France we were directed to use a rough mountain path at one stage, of the type that would be a challenge on foot. But that's a weakness of the mapping, and despite having better coverage, the Navteq maps seem slightly inferior to the Teleatlas product (as used by Tom Tom). A quick press of the nifty "Detour" button eliminates rogue routing so it's not a real problem.
Also, if you've changed settings to request the shortest route to a destination - don't forget to change back before your next journey (it isn't obvious that this is set) or else you'll be touring tiny back streets unexpectedly!
There are lots of pre-loaded POI's: sometimes it's a little awkward to see these on the map. It is easy to load custom POI's yourself (I do this a lot) using a PC cable (you may have to buy one of these, but it's a standard mini-USB which many digital cameras use: so check whether you already have one). The feature "go to coordinates" (where you enter the Lat and Long) is a great boon: I use it in conjuction with Memory Map on a PDA, and I can then be directed to the exact point on a detailed map.
Overall, pretty good if you like portability, simplicity and ease of use.