It lasted about 2 days before someone tore it off my bike while it was parked. A pity, otherwise I would install the aftermarket speedo like you said.
Some people around here use an aftermarket speedometer and remove it from the bike every time they park. Personally, I don't feel the need to know how fast I'm going. I do like to know my approximate average speed, but I do that by checking a clock before and after the commute and using
http://maps.google.com to figure out what the distance traveled is.