I have both a real Thermapen and this cheap copy:
I got worried about dropping the Thermapen in a pot of water, so I bought a cheap backup. The Thermapen is definitely nicer, better made, and faster, but the cheap copy is within 1 degree of the Thermapen. It takes a couple of seconds, as opposed to being almost instant, and I have to switch it from C to F every time I turn it on, in addition to pushing the power switch.
I end up using whichever one is closest at the time.