Are they worth having? Are they actually any good? Or a waste of money?
I've found my Tom Tom invaluable on many occassions. Especially good if you're on your own, not having to stop all the time to look at the map/A-Z.
They aren't fool proof, and they will occassionally give out slightly dodgy info.
I tapped in a postcode once for a remote house out in the Shropshire country side, and it sent me into a field full of sheep...2 miles from where I should have been.
Generally speaking though, yes, get one. If you travel europe, you need that mapping too, so don't forget to ask. They haven't all got it as standard.
Tom Tom is very good, and the latest model is slimmer than the one I've got too.
After market service is apparently pretty poor so I've read. Never had to contact them myself though, so I can't confirm that.
Updates available on line.
Do the usual googling and product searches before purchase to find the one that suits you best.