I would never set off from home on a trip without one Patboy. People will tell you that you can buy a lot of maps for the money ... yes you can, but they miss the point.
People will tell you that they like 'getting lost' ... well you still can, turn it off. Fact is I've found some cracking motorcycling roads by using the GPS. You can download routes you've created at home using the computer, and a good map alongside, to use at a later date, on a tour etc.
My advice, no doubt others will be along soon to proffer theirs, would be to get a Garmin... what they don't know about GPS's isn't worth thinking about. Buy second hand by all means, get the best/latest that you can afford. With the new Zumo 660 just coming out then no doubt there will be plenty of 550's up for sale, which would probably be your best bet.
Buy a new Zumo 660 and you'll be set up for a few years.
I run the BMW Navigator III+ and it's great, but maybe somewhat outdated. Mapping down to street level for the whole of Europe ... BUT I have a S/H Garmin Quest (£100 off e-bay) for using with the Word Map, or Topographical maps of the UK and Morocco.
As a f'rinstance, went to the Pakistan Embassy in Bradford today for our Pakistan Visa's. Put the address in and it took us straight there, to the door, how clever is that
Jump in at the deep end and buy