I used a Garmin GPSMap 62S for a 4-day road trip to the Catskills and back. I had all of the routes pre-programmed with routable maps downloaded to it. For the most part, it worked really well. Sometimes it's a little slow to update so I did miss a turn every now and then. I noticed that for directions it's definitely not as quick to update as a dedicated automotive GPS. There's also no bluetooth or spoken instructions.
I have a RAM mount on my handlebars and I didn't have any trouble seeing the screen. The transflective screen is great in the sunlight - very readable. The backlight allows you to see it easily at night as well (although I haven't tested that yet so I don't know if it's too bright).
The main reason why I used this GPS was the ability to program routes. The GPS then keeps you on the route. An automotive GPS doesn't do this, at least my 8-year old TomTom doesn't.
The 62S also goes on sale periodically for around $200. I bought mine at Sail.
Whichever GPS you decide on, make sure that you can get maps for the area you want to ride in. If you want directions, then you have to have a routable map. If you just want to see where you are, then a good topo map will be useful - this will also be useful for hiking, canoeing, etc. Backroad Mapbooks sell the highest-detailed topo maps. That's what I use for my trips to Algonquin and they show everything.