I've been using this workstand lately and find it to be pretty useful for everything but the most difficult tasks (like cranking on "removable" BB cups). For most tasks, it's pretty handy. All my steel tube bikes fit nicely.
I've tapered the tire tube so that I can push the bike into the stand and it stays pretty firm. I am thinking about some kind of clamping gizmo that will not allow the bike to pop out under any circumstance, even the BB scenario mentioned above.
The inspiration was in Instructable called "Workmate Bike Stand" which confused me a little at first...
My stand is clamped into a pretty solid bench vise. No worries about getting loose (but please don't try it with a clamp-on vise). If you have some exposed studding, you can probably just use lag bolts to fix it to the studding.