when you build a table top you will usually do a big slab surface of some type. Either get some thick, heavy stuff that won’t sag like thick lumber (like most workbench tops) or you use thinner stock and add braces under it to prevent sag and flex (like most counter tops where the cabinets act as braces). a torsion box is basically made of a thinner top and bottom pieces and has structural ribs in between. This enables the top to be both strong, inexpensive and light at the sacrifice of time. Ikea uses lots of torsion boxes in their designs because they can use exceedingly thin surfaces that are ecologically conscious, light and strong. Many of their side tables are basically a cardboard honeycomb with very thin veneer around it (just a few mm)
for my torsion box, I have top and bottom surfaces of 1/2" mdf that are around 48" square, I put a set of strips of 1/2" mdf sandwiched in the perimeter of the surfaces and then cut cross members that cross each other every few inches in between the perimeter pieces that create a grid inside. The thickness (height) of these middle pieces will determine how strong the box is overall, I eventually settled on around 2", though thicker is stronger.
Theoretically you could skip the bottom surface but in my experience it really adds lots of rigidity. Some people also fill the voids in a torsion box with foam. this only marginally improves rigidity but does wonders to reduce resonant frequencies of the box. I elected not to fill voids and have had no issues with resonances (they won’t impact carves, but just add to the noise)
this video should give you a good primer: http://www.thewoodwhisperer.com/videos/episode-18-assembly-table-torsion-box/
This thread should give you lots of ideas as well (including pictures of various torsion boxes in various stages of assembly): X-Carve Workspace Showcase
To your other question; if I had the space and hadn’t already built my station I would probably consider it. The design of my station has a rack under the x-carve and I’m just going to put the x-controller on a pull-out shelf when I get it… I’ve only been waiting since May…