I have had one for quite a while, and use it for cutting down sheet goods. I have the 4' x 8' size.
It is very stable on a flat surface. i haven't tried it on dirt or other surfaces than concrete slabs. I use it in my garage shop. They have a video on their web site showing it holding 3,000 lbs. of concrete block, if I remember correctly. Anyway, it is very sturdy.
I put a sheet of 3/4" ply on it, and a hard foam sheet on top of that as the surface to put the sheet I'm working with on top. Works great. They keep coming out with accessories, such as risers and clamps that work with it.
Steve