My experience is limited, but I did buy one.
The little I tried to use it, I found that you need to be quick about getting it moving once you flip the switch on the side to let the fluid flow. The flow of fluid through the sponge is pretty quick (maybe the lever can be adjusted, I don't recall if it's all-or-nothing).
So if you fill it up, flip the switch and then get around to hooking it up to an engine to pull it along, you may have significant amounts of fluid deposited while it sits waiting. Make sure you have everything else ready to go so you can flip the lever and then have it moving pretty swiftly.
Hopefully others can chime in with more experience than I described above.
-Dave