At my work, we sell 2 gallon drum bowls, and I have one as well. The way that they are shaped, they have a flat back and front, with curved sides, which allows for a heater and the right filter to be applied. At work, we use the Tetra Whisper 3i, which easily suction cups to the side of the bowl, and a heater, to help fish who are not doing so well in their cups. We add plants and hides, and we use it also as a way to show customers how easy the set up is, and how much better it is than the 0.5gallon bowl they are looking at.
With frequent water changes, these fish do great in these 2 gallon bowls - happily swim and dance, eating like champs, and building bubble nests.
My 2 gallon bowl is currently empty, as I put its inhabitant into a divided 10 gallon to try to have a few tanks/bowls around the house as possible, but I keep it around in case I need a hospital tank, or if I find another betta that I really like and need a place to hold him until I figure where in the house he is going to go xD
The only reason I do not care for bowls is the fact that they are hard to do water changes with. The larger bowls are often heavy, especially when filled with water, and if you hands get wet, it's hard to hold onto the bowl.