It is a useful technique, but I have to admit it does take practice. I've been shaving with a straight for almost a year now ( has it been that long already??) and find it a useful stroke but it took some time to get right. But, only a few weeks ago I got a good sized cut right on the underside of my chin from doing a slicing motion, as I had too much sideways motion and not enough downward motion...

It's important to do a nice smooth stroke as well as keeping a very light touch, so if you do hit a snag you won't dig the razor into your face.