The Garmin fans will point you towards Garmin, the TomTom lot will back their horse too. There are others, but you won't go far wrong with anything from either of those. I went Garmin 1300 series last summer, GB & Europe mapping. Bought USA/Canada maps on micro SD for a holiday which just slotted in the side - all worked flawlessly. Think I paid around £89 for the Garmin unit - Amazon prices were pretty good.
Get a Tom Tom XL2 IQ routes. Currently at Tesco for £99.97. If you fill up with 20quid of fuel while you at Tesco you will get a voucher for 20 quid off any tom tom. Makes it 79.97. Bargain. Lane recoginition on it on tells you street names ect you turning onto. Only UK maps though. Have to pay extra if you want europe!!
A lad I work with who does a lot of travelling around the UK and Europe in his camper van has a Garmin 1300 and he reckons its the best Sat Nav he's ever had.
Oh, and worth the money alone for the pronounciation of 'Osceola Parkway' it came up with in Florida....
Mine is a cheap one £50 I think it cost, it's a binatone one. Does the job, although the battery on it is shocking, lasts about 30 minutes, so I just plug it in to the cig lighter when I get in. Oh and it's the 2nd one i've had as the first one packed in after about 3 uses.
Tom Tom all the way for me - use the same type at work and personal. I got it from Argos with the carry case for £75 just before Xmas. I tried the Binatone one that Asda sold last year and had the same problem with it packing in after using it for a week!