My experience has been a bit different than Daboos. The Magellans (three) I had for cars were poor. Tried to cram too much information on the screen and resulted in one that was hard to read.
The Garmins I have had (two) are much better and do a good job for everything I need for a car. Screen is OK for a bike, but, still can't change the color selection. As Daboo said, some have weird power plugs that only work with the car mount. Some of those you can wire yourself but it involves cutting the cable or getting a new cable to cut up.
I have only used one TomTom and didn't like it at all. Screen layout or menus or how it figured directions. But, thing change so don't know what the new ones are like.
The CoPilot system I have for my phone is best bang for the buck. $30 with lifetime map updates. Same software also runs on my tablet so I can set up a map and plan a route, then send it to my phone. For directions it is pretty good. Yep, there are times when it lags behind a bit, but, that doesn't happen very often. And some areas have weak coverage for roads but that is getting better. You can also change the screen colors so the road stand out better when sun is shining on the screen. Not perfect, but, it means I only have to carry one thing with me instead of having a phone, iPod and GPS.
I also tried a sunscreen on the bike. It worked, kinda. What I found is better is a RAM mount. Tilt the unit so the sun doesn't glare off of it (or your jacket, or the tank bag
). Put the unit on the center of the handle bars. Yes, you have to look down at it. But, it also means that the sun won't shine directly into it from behind you like it does if you mount it up high on the dash. Yep, BTDT too.
If the bike specific ones have a screen you can read in direct sunlight then I'd find a used one, but, I'm not going to spend several hundred dollars for a GPS unit and never over $500.
But, I am a cheap bas--- too.