I just got one of these recently, and put it on my MTB. After 7 miles on extremely rough terrain, it held up well. Waterproof.
Wireless Cycle Computer Bicycle Bike Meter Speedometer | eBay
-Cheap, cheap, cheap.
-Fairly easy to install. Includes batteries and tie-wraps.
-Pretty damned accurate.
-Has many features
-Easy to read
-Did I mention it was cheap?
-Had to use a small screwdriver to align the battery contacts to make them contact the battery properly.
-Waterproofing gasket is very thin and fragile.
-Not exactly snug on the clip holder.
-Like most cheap electronics from China, instructions require some thinking between the lines.
-Does not support the U.S. economy.