I have a zumo660 .. nice big screen & easy to use even with gloves on, the only drawback is that it doesnt lock onto the mount ... Mr_P has a zumo550, which does lock onto the mount but is a smaller screen
used a zumo 550 (garmin) for last 12,000 miles or so, dropped, soaked, forgotten, shouted at, no probs, bitfiddly to up date, music good, easy to use.
pick the way you intend to 2listen" carefully, i use moulded silican ear plugs with speakers, saves expensive interface like autocom,
tho not cheep themselves!
have heard that from april this year sat navs on "handelbars" will be illegal in europe, so pick something you can get a propper mount for?
what el ninja said usually,
but it is nice sometimes (like 2am half way across rain soaked italy/france) to just know how long it is to fuel/hotel, and when the dreaded roar works arrive where else you can get.
holding a finger in place on the map, trying to navigate, at motorway speeds became less fun for me about 12 years ago.
(opps, out loud again?)