I just received my PumpToys VelSeal cylinder and tried a 1-hour session with it (using a water-based lube), taking breaks every 5 minutes.
It has an excellent fit and finish – the cylinder has a universal female coupler at the top. The VelSeal works perfectly... it makes a good seal and stays comfortable, even after a long session. The etched ruler is handy as a guide for your progress.
I cleaned the VelSeal material with a soft, wet cloth. The website says it is relatively inert against most chemicals, but I'd rather not take chances.
When buying a cylinder, remember: get the right size. Too wide will never seal well and it will pull at your scrotum. Start with a cylinder that is almost exactly or only slightly bigger than your full size. With the VelSeal, figuring out the proper pressure was never a problem... I could easily tell when I was reaching the right level of vacuum.