Well I bought one also. With the Festool sanders you really need to secure smaller pieces down to keep them from moving. Clamping from the sides was desirable as the clamps would not have to be moved to sand the entire surface. Festool sells a pair of side clamping clamps for about $90. I was reluctant to take the plunge and kept putting that purchase off. I have been using rubber router pads but they were not quite enough any more.
I bought the Vacuum Clamp from Woodcraft this afternoon and to add to what Robatoy has mentioned, the clamps do work well on porous Oak. To check the holding power I set the vacuum clamp, which is about 5.5" square, on my TS top and turned the clamp on. Then I placed a 2' x 4' piece of 1/2" Baltic Birch up on the clamp except I sat the panel on the clamp on the corner of the plywood panel. All but the corner of the panel was hanging ff the edge of the TS top. Pretty impressive.
Next I set the clamp on a steel top work bench, turned it on, sat a 5' long piece of 1x6 Mesquite on the clamp and again at the very end of the board. I grabbed the opposite end of the board and moved it up and down. The steel top of the work bench flexed but the clamp held fast.
Pictures of the last test.