Has anybody tried this, does it work and how secure is it?
Yes, not really, and as secure/private as you'd expect Google to be ... i.e. don't upload stuff you wouldn't want in the public domain (Google tend to use information from one system to develop features in others... so there is the very high chance of, at the very least, a handful of Google employees seeing what you've uploaded).
But the Google Image search itself (as opposed to the reverse search you are talking about) can sometimes be surprisingly good, unfortunately not always though, its got some way to go before it reaches the heady heights of its text search elder brother !