I think hiking Jim did a blog post on refilling (maybe the guy in the video). I will say if you know what you are doing it is not terribly unsafe. The tanks are not made to refill so they won't last as long as those big refillable propane tanks. The worst that can happen is you end up with a damaged valve the butane leaks out and evaporates. Leaks are always a potential issue so you should be sure to store your tanks in a safe manner anyway (well ventilated, away from other flammables and flame sources).

Yes, pure butane will not have very good low temperature performance, so a refilled cannister will only be good for warmer weather trips. Add that to the equipment and large butane canisters you need I have never thought too seriously about doing this.