I've been using a Garmin Forerunner 300 which I bought used from Ebay the other year.
I'm quite impressed, it plots your course at regular intervals and after the ride you can download the info to your PC where it shows you the route on a map, or you can view your course in Google Earth which is quite cool.
It records your HR if you're into training (can be used for bike or running), and as its GPS it also does Speed,Avg Speed, Distance,Altitude,Gradient etc etc... so I have never bothered with a regular bike computer since.
The watch itself is quite chunky, but not noticable when riding.
I'm sure there must be far better systems available now, but 2nd hand from Ebay it's pretty good for the money