We used to use one at the co-op I volunteer at, whenever we cut forks and handlebars down, but we went through an awful lot of cutting wheels for it. They also, in my experience, have a tendency to move sideways across the surface rather than cut into the steel, since steel's much harder than copper. Ours was size-adjustable though.

We now have a Park Tools cutting guide, but in all honesty, I normally just put the soft jaws in the vice, stick the fork in it and cut it by eye. I can get a fairly straight cut by doing that. I also file a taper onto the top of the tube, rather than leaving it flat. I find the stem goes on more easily that way.