Bottom feeders give you advantages similar to dripping, without the hassle of actually dripping. The bottle is effectively attached to your atomizer/cartomizer from the bottom, through the 510 connection. When you would otherwise drip, you just give the bottle a small squeeze (jargon term is squonking), and that feeds the liquid into your atomizer. One of the nice things I've noticed is that cartomizers seem to taste better this way, since juice gets fed directly onto the coil.
There is, however, a bit of a learning curve involved in not overfilling your cartos/atomizers, and not letting them run dry.
The three most popular bottom feeders are VMod, Reo, and Bogger Box. Reos are my favorite, but here's some quick pros/cons
1. VMod is cheapest, both in price, and in quality. It does not natively work with rebuildable atomizers, though it's possible to get it to work
2. Reo is nigh indestructible, very well made, and more expensive than VMod. Works with a lot of rebuildable atomizers. Very good mod, great support community. The indestructible case in point -- some guy has dropped his a number of times while riding a motorcycle on a highway. Now it's fairly banged up, but still working without repairs.
3. Bogger Boxes are wood. Similar quality and price to Reos. Arguably prettier, but require gentler handling, as they're wood instead of aluminum. They also work with rebuildables, I believe
Reo Grand and Reo Mini
VMod XL
Reo Grand and VMod XL