I have and like my InterAct MakoPad PC, which is identical to the PC PowerPad Pro except for color and the name on the top -- it's black instead of blue; both are model SV-234. I had a very difficult time getting it to work on my Windows 98 SE (w/ some WinME components) machine until I happened upon the ONE driver website that has an older (probably the first) driver for it. There's also a very hard-to-find patch for it that I picked up, but the patch crashes my system just like the common driver version. I'd upload the files here but they're all much larger than this site's miserably tiny upload limits.
BTW, there's more than just "an additional button" unseen in the auction photo you linked. Shown on the face of the pad are 6 buttons, A-B-C-D-L-R. Not visible on the shoulders there are duplicate L and R buttons. Lower on the left, where your middle finger should naturally rest, is yet another L button. On the right shoulder below that R button, is a throttle control. Unfortunately, this controller can only activate either the L and R button sets or the throttle control, not both at the same time (you select either "6-button" or "4-button w/ throttle" under the Windows control panel, as well as a switch on the controller itself).
Also perhaps not obvious from the auction photo, the thumbstick and cross-pad are really the same control; you select which to activate by another switch. Naturally the cross-pad is digital only, but the thumbstick is true analogue.