Aye GC,
Use these all the time at work to record soil pits, navigate about strange bits of hill, woodland, forest etc. Get covered in mud, used in rain, dropped bounced, squashed, etc.
Garmin GPS12, solid, 'old' tech, 'brick' of an instrument, but good enough to use once you've arrived at a town/village, to haul out switch on and 'GOTO'. Plenty of re-furb, 2nd hand units + accesories on Fleabay ( your favourite shopping experience!).
No longer any 'new' updates from Garmin site, but cheap and rugged. Don't 'see' the satelites to well from under trees or in buildings.
'Garmin12 link'
Another model which is cheap and good but still has the viewing satelites weakness is the Garmin 60 GPS.
'Garmin GPS60 link'
If you have more dosh (HMRC!) then the Garmin 60Csx is the preferred and obligatory instrument if you want to win contracts. All the bits and pieces and the ability, because of it's aerial design, to get sub 10m accuracy beneath tree canopy. Use this always these days in the job. Updates to software from Garmin still available.
'Garmin GPS 60 CSx'
Manufacturers website: 'GARMIN' This will drive you 'potty'!! Simon's 'porn' site!!!!!!!! 
My regards, Bill.
Bill, Do these Garmin's have road mapped navigation capability? I think these would be fine for geocaching or off the beaten track where you need accurate waypoints, but I think what Graham's after is an automatove sat nav with features such as postcode search, turn-by-turn navigation, POIs, etc.
I've used a cheap £50 Navigo sat nav from Ebuyer for the last few years and hacked it to replace the Turbodog software with TomTom and igo8. I protect it from the elements with an Otterbox (2nd hand from Ebay) connected to the handlebars with a RAM mount. This gets around the problems mentioned by others with moisture and vibration, but the cold can also be a killer and the unit hasn't been as reliable the last couple of Eleffant/TTs.
The dedicated motorcycle units from the well known brands out there are IMHO overpriced and from what I've seen don't always deliver seeing as they're supposed to be bike specific. I'm thinking here of the failed power connectors on the TomTom Riders. Jethro's brand new Garmin unit also had power issues on our recent trip. It seems to me you don't seem to get a decent biking satnav even if you're paying for it.
Saying that, I know SteveD swears by his Garmin Zumo (550?), but this is quite a pricey unit. Personally, I think I'll be sticking with a cheap car based unit in an Otterbox, but I did recently come across the following bike specific one that looks interesting:
http://www.pixmania.co.uk/uk/uk/6879598/art/mappy/mini-330-gps-for-motorbik.htmlMobile phones, especially with built-in GPS receivers are an other option. I have used a mobile a number of times when my dedicated satnav has been playing up, but the small screen size can be an issue when on a motorcycle. Also, unless it has a nice big touch screen, it can be awkward to use with gloved hands.
The other thing to bear in mind is that the majority of the satnavs and mobile phones have a very short battery life so a power supply from the bike's battery is a necessity unless you're doing very short journeys or you're disciplined enough to put the device into standby (if you can do so easily with gloves) for long sections of road, such as on motorways or major A roads, where you know there isn't going to be a turning for a long while.
HTH,
Simon