I intend to write some more formal thoughts on the Dakota 20 and the custom maps ability sometime this week. Screen viz is an issue for use on a bike, but not if the unit is in hand. I just got back from 5 days of trail surveying (on foot) and the Dakota was great... for the most part. Having actual USGS quads on the GPS for the area we were working on was very useful.
I don't know if it's only USB 1.1. Transfers of large custom maps don't seem slow to me. As for wireless, I have not used it, but I think it is only to talk to other Garmin units. There's a regular old USB-B connection so the unit uses the same cable nearly all Garmins use. No dongle needed.