I've owned two of these and ridden in all 4 seasons, driving rain, blinding snow, and even the occasional sunny, warm day. I've never had a problem with either, and find them very easy to set up and calibrate, easy to use, and totally accurate, dependable and reliable.
Under $10! Can't be beat! Ride on!!
Update 8/2013: It is a little over $10 these days, but still a great little unit!