Originally Posted by Maelstrom
Your bike will be shaking and rattling on dh runs. Unlike xc runs where the movements aren't so jarring. I don't know the type of dh you are doing but the bike works hard on runs. A ht is even worse for being jarring. the computer monitor will...and I mean WILL spin, the screws can come loose and it CAN fly off. Those quick release ones WILL break off. Not to mention crashing.
Weird. The computer was the ONLY part on my old freeride bike (the old RM6 from '00) that I still have. It faithfully recorded each of the 15,000 km that I logged on the bike, and I never had any problems with it. It was a 1994-vintage vetta C500 wireless unit... I moved it to the new VPfree, it still works fine. I dislike riding without it.
Granted I ride a lot of technical XC on my big bike, and only do lift serviced DH a couple times a year, and blue mountain is not whistler.
My brother though had his speedometer rattle apart just riding to work. I guess you have to shop around.
Another higher-budget alternative is to put a GPS in "record tracklog" function and stuff it with some padding in your camelback. In addition to recording all speed functions, it will map and give vertical profiles of all your trails.
Of course, to see your speed in real time you'll need to mount the GPS on the handlebars, which is NOT a good idea for the reasons everyone else is giving.