I use a PDA for tracking contacts and keeping my schedule on it and my old one died several months ago so I went to replace it and decided to get one of those GPS ones that have the mapping software on it (from Navman). The price was very good (comparative to the non GPS versions) -- I picked it up for less than $400. I have been extremely happy with the unit and using it occasionaly for directions to an unfamilair place has worked great.
One of the things that I like most about it is a program I picked up for it called Intelligolf. The program acts as a scorecard and tracks just about every statistic possible golf-wise. Scorecards for pretty much every course are available online.
The version also works with GPS and it can be used to calculate drive distances as well as telling you your distance to the front/middle/back of the green. The GPS data for most courses is not available, but you can record this data as you go on your home course (stand at the front of the green and hit a button). I rarely ever use the GPS functions and usually just rely on the fairway distance markers (don't want to spend the time recording all the GPS points).
Entering scores and data into it is very easy and can be done between holes. I usually only track score, number of putts, drive distance (program calculates this based on how far you are from green) and location, 1st putt distance and result -- sounds like a lot but everything can be entered quickly on 2 screens. I may start tracking up and downs and sand saves. It will automatically detemine GIR's.
When you are done a round it is fun to see all the stats for the round and it can help to show you areas for improvement. I don't think it will make a huge difference (or even a small difference) in your game, but it is a fun gadget to have.