I am not an experienced pumper, but I bought a conventional type (dry) vacuum pump and cylinder recently and have been playing with it. I have not used the Bathmate, but I have read a number of threads posted by guys who have. A reasonable one can be found here:
http://www.thundersplace.org/forum/showthread.php?t=131476&page=1&pp=15
Unfortunately, since that thread contains some penis pictures, it is located in the "members only" section of that forum. There are also extensive threads over at the M*****s of S*ze forum, but since that forum is a Bathmate "partner" they have a financial incentive to push it over there.
I have heard of guys using a conventional cylinder like those made by Vacutech for water pumping and applying suction to the tubing by mouth. I think I also heard of a guy water pumping with a gauged pump who would keep disconnecting the tubing from the cylinder while pumping, and allowing the water to drain from it before reattaching it, so as to keep water out of the pump itself.
I have also heard of guys using the Bathmate for air pumping (as was discussed in the thread I linked) but it didn't sound as if it worked too well. I think if you want to water pump the Bathmate makes more sense, and if you want to air vacuum pump a conventional cylinder and pump make more sense.
The advantages of the Bathmate seem to me to be:
1. Well-suited for water pumping, if that is what you want to do.
2. Simplicity. No need for an external pump and tubing.
It seems to have some significant disadvantages to me, however. These are:
1. Not well-suited for air pumping, according to what I have read.
2. Use for water pumping largely restricts use to bath time. If you use it in the shower and let the water run, you are going to be using a lot of hot water if you pump for 20 or 30 minutes.
3. A very limited selection of cylinder sizes. It only comes in two sizes and from what I have read, if you have a maximal girth of 6" or more, the regular Hercules is probably not going to be wide enough for you. The larger Goliath sounds ginormous.
4. Lack of a pressure gauge to monitor and adjust vacuum pressure. The Bathmate is pressure-regulated by a valve, but basically you pump it up until water stops coming out of it and that is it. Furthermore, I have yet to hear exactly how much vacuum pressure that is. Perhaps someone here knows.