I'm thinking of getting one of those in-car GPS navigation systems (eg Tom Tom). Anyone here have any recommendations/experiences? Have they proven genuinely useful to anyone or can they be an annoyance?

(This is in the UK, btw). Thanks!

"Have they proven genuinely useful to anyone or can they be an annoyance?"

Yes, and yes. :)

If it's aftermarket, I expect it to be quite useful (otherwise TomTom wouldn't be a major player on the market). Some of the built-in ones have been crap, apparently, particularly the BMW ones.

I have an iPaq with a Garmin GPS unit attatched, and it's come in handy once or twice. It really depends how much travelling you do to places you've not been before. Mine has sent me on wild goose chases before; once a road had been made one-way but the map data hadn't been updated, and once or twice it's sent me the long way round a town centre, but on the whole it's quite handy. (I would recommend checking a map before you leave and getting human-generated directions (landmarks are always handy) though.)

Get the one from family guy with the Yakov Smirnoff voice!

"Turn right at fork in road. In Soviet Russia, road forks you!"

If you have a laptop and don't mind something bigger such as that, MS Streets and Trips that comes with a little USB GPS unit you suction cup to your windshield is freaking awesome.