I use a Garmin GPSMap 60 that I got while serving in Afghanistan. I'm married to the thing. Anyways, I ordered the handlebar mount for it, stuck it on and use it every day while commuting, as well as longer trips. In addition to having turn by turn directions you have any kind of speed data imaginable. Moving average speed, moving time, MPH, etc, that are all totally customizable.
I had seen a touring website a while ago that said that GPSs are worthless to a bicycle tourist. Couldn't be farther from the truth. While nothing can take the place of a good paper map, if you're willing to spend a few bucks a good, hand-held GPS is worth it's weight in gold.

Just my $0.02