LOL @ Lil O'lady
They are handy if you are traveling to an area you don't know, they may not take you the most direct and logical route that a local would take but they will normally get you to your destination OK.
If you get a tom tom then you get all the extras that you need to use it straight away with it, the only other thing you might want is a protective case to keep it in when you take it out the car.
You will be able to get them cheaper on line and as long as you stick to the well known sites to purchase it then you should have no problem, I'd recommend that you use a credit card and not a debit card for online transactions as well as you get some added protection from credit cards if there is a problem.
I would personally recommend
http://www.amazon.co.uk as I have bought lots of things from them and have only had a problem with an item not arriving once, I contacted amazon about it and they refunded me straight away with no hassles at all.