I use a Bissell Powerforce Bagless Upright Vacuum. (dedicated to poo's? yes)
Bissell PowerForce Bagless Upright Vacuum Cleaner, 65792 - Wal-Mart
Image attached below.
I like it just fine, and it has worked perfectly well for me.
It was a modest 50.00 at walmart.
The chamber isn't as small as a hand vac so I don't have to stop and dump before the cage is finished. (this is important with 7 pigs !) but it isn't so large like a shop vac that it holds a ton of poop and is harder to dump. The chamber just slides right off the front so I can dump quickly and easily. The filter on it, is washable, and very cheap to replace (havent had to replace it yet in almost 2 yrs). It has a seperate chamber where the filter is, so I am able to place carpet fresh crystals in there so the vacuum blows "cinnamon" rather than "pig poop" smell when I am doing my suckin' poo.
(if i place the crystals in the regular chamber I have to dump them with the poo--so a waste of money)
Since it is bagless, and everything sucks straight through the hose into the chamber, and the chamber detaches to be cleaned/rinsed out with bleach...I haven't had a problem with moisture harming the vac, nor with mold.
There is no way I would use a regular bag vacuum, but this bagless one has been great.
The one draw back, is it will suck the fleece clean out of the cage if it isn't clipped down on all 4 sides. hehe But this hasn't really presented a problem for me since I have to keep pigs from burrowing under the fleece anyways.
I actually own a shop vac also, but It takes up so much space..It stays in the shed. The upright vac can be slid behind a door, or in a corner.