I was using a Woodcraft clamp-on type at the beginning of the season, but I had it attached to a custom bar-end because the bar wasn't long enough. It tore out in a crash. Then I had one of the Chinese ones with the expander in the end of the bar. It was never as secure as the Woodcraft (i.e., you could rotate it pretty easily), and it also tore out in a crash. In that case the exposed end of the clipon ran across my finger and tore it up so bad I ended up losing part of my finger.
With that experience, my objective is to have a setup that maximizes the likelihood of the brake lever guard doing its job, but also of the bar end staying in so it doesn't leave the end of the clipon exposed. Current setup is a Woodcraft clamp type guard on custom (i.e., long enough) Woodcraft bars with the Woodcraft bar ends installed.
What I am saying is that the lever guard is important for safety, but so is a good secure bar end, so don't sacrifice one for the other if you don't have to.