The Zyliss vice is very versatile, especially with the additional extras available for it, it's portable and can be set up anywhere. It's the Swiss army knife of vices. On my 8'x4' bench it can hold pieces about 7' long when used upside down. I've not had a problem with it working its way loose when in use, though I wouldn't say it is for really heavy duty work. Makes a good prop for holding things up when attached to a batten too.
These two YouTube videos show most of its versatility, though this is an exact copy of the original Swedish made Zyliss Profi-King, but all components are interchangeable between them.