I've been using the pole mount for a couple of years on two different vehicles. I bought mine before the mag-mount version came out, so on each vehicle I had to rig a special bracket to mount it.
I've tested it using my speedometer on calm-wind days, and the margin of error is consistently only +-3mph. The vehicle's slipstream influence is actually not that significant with the Inspeed mounted a few inches off of the roof. This seems to go against aerodynamic principles, but for some reason, in practice it works OK.
The highest stationary wind readings I've measured with the vehicle-mounted Inspeed include 114mph in Geary, Oklahoma on May 29, 2004 and 102mph in Fort Pierce, Florida during Hurricane Frances' eyewall in September of 2004. I've seen it hit over 120 a few times while driving 75mph into strong headwinds in South Dakota.
The bearings will eventually go out on the rotor if you leave the unit installed on your car all the time (which I used to do, subjecting it to long periods of +80-90mph readings while driving on the interstate into headwinds). When the bearings go, it will result in vibrations that reverberate loudly through the roof of the car. So, I'd only install it on chase days to extend its useful life.
A nice thing is that you can leave the unit on the car while you are parked at work or at home for an extended time, and it will record and display the highest gust that occured during that period. It's also nice when stopped observing a storm - you can reset the highest gust indicator and leave it while you go watch the storm, then come back and see what the highest reading was without having to hold a Kestrel the entire time.
Also, the guy that makes these anemometers is great to work with and has sent me replacement parts free of charge. Just as a disclaimer - my old setup is pictured on their site, but I'm not a 'paid endorser' or anything of that sort. It has just worked well for me.
