The Omni can be used as a chair to transform any corner into an impromptu mini-lounge (just add martinis). It is surprisingly light for its size, and is big enough to comfortably suit two (Sumo says that it is "chair + lounge + loveseat + floor pillow", something that everybody here agrees with).
I haven't closely followed bean bag technology during the 20th century, but something must have been recently discovered to allow this bag to be so light, yet to go from soft and mellow when laid out (much better than a Futon ("uncomfortable sofa by day, uncomfortable bed by night", as a comedian used to say)) to firm and supportive when folded upon itself.
The Otto is less versatile in form, but has the advantage of being able to replace a low chair, or serve as a footstool. Like its bigger, more pliable big brother, it is very light, yet comfortable as a mushy-firm chair.
Ergonomically speaking, I've read that back support is usually cited as an important feature of a good chair. From personal experience, my back feels better when I spend time on a "backless" chair; I've heard a physical trainer say that spending time on a exercise ball was actually good for your back since you were constantly making small moves, that some back muscles had to constantly compensate for. To each his own.