You will not get much love on this forum for pony bottles as most tend to believe that if you are concerned about such things you should be diving doubles which actually give you a properly redundant system. Marginal increase in weight but you don't have to have 3 tanks, only 2, don't have to worry about filling that 19cf bottle, and everything is working regularly so that is likely what you will see from everyone responding.
Edit: I will give the disclaimer that they are obviously used in the commercial/PSD world when diving with FFM's and make use of switch blocks and what not so the diver doesn't have to share air and remove the mask due to concerns with contaminated water and what not. This is outside the scope of recreational diving and while I think FFM's should also be outside of the scope of recreational diving people seem to like them. To me they are just a necessary evil when I have to talk to people either in-water or at the surface.