I have a 20 gig Creative Labs Zen Vision in my GL1800 and my wife has a 20 gig Ipod in her GL1800 trike. Both work great. The software that comes with the Ipod is better than the Creative Labs software, in my opinion, but there isn't any difference in how well they work. They are virtually the same thing built by different companies.
If you are thinking about a GPS in the near future I would think about getting a Unit with MP3 capabilities. I have the Garmin C-550 and it does a great job. It even has bluetooth for my phone. All in one units means less clutter on the dash. Even though the c-550 is not waterproof, I had a second skin cover made for it so now i don't worry about a little rain.
Main reason is that's it's almost designed to operate with gloves on. The whole screen is a rocker switch that controls all the functions. Being solid state there's no disc drive to worry about and it plays for about 30 hours.
20 Gig RCA Lyra. Piece of crap. Avoid RCA. Tried foam rubber and helped a little, but once it gets bounced around a little, it may stay silent for 5 or 10 minutes (or until you come to a stop) before it decides to start playing again.
My Lowrance iWay500 GPS is also an MP3 player, and I have it connected to the AUX audio plug in the left pocket. I did install a Kennedy ISO Set ground loop isolator in the line because I had some alternator whine when playing MP3s. End of problem.
Got it from Wingdreams.com, their item number 95332. $27.00