These actually work quite well, as long as you use a relatively small plate. Some of the hooks are made of a really cheap material though and won't last long(usually the cheaper ones). I found a guy with some scrap rebar who made me a couple of these.
That's definitely a good inexpensive holder... I use them for pistol steel. For steel rifle targets I use an old kid's metal swingset I got for $20 on craigslist. Works amazing, and I can break it down and take it to my shooting spot very easily.