I've heard of the positive pressure oil filled things, but usually in connection with problems.
Bearing Buddies work pretty well as long as you don't try to overfill them.
To use them correctly, pack your bearings as normal. Fill the inside reservoir of the Bearing Buddy with more grease (this will be at the slack position), and install on the hub (a piece of 2x4 and a mallet should help keep it lined up.) Then, using a grease gun, add grease to the nipple until the cover plate is a little shy of topping out on the outer rim. If you pump more than that in, you will blow out the rear grease seal, opening up an avenue for grease to get out and water to get in. The extra space can be considered expansion space, as well. Use a cover on the BB to keep the grease nipple clean (or cleanish, I guess.)
One other thing I do is monitor the hub temps while towing. Get a remote infrared temperature gauge (about $40 at Radio Shack) and shoot the exterior of the hub whenever you stop. They should not get any hotter than warm to the touch. Mine usually run right around 100 degrees F. A rise in temp means someting is causing friction in there somewhere, and needs to be looked at as soon as possible.