I have a back mounted 19cf pony. I attached it by placing 2 hose clamps around it to hold a metal bar, and then slid a tank strap between the bar and pony. The whole thing just slides right on to the main tank like you would attach your main tank to your BC.
I really like it. I don't notice it in the water at all. It has basically no affect on dives and is only marginally heavier on land.
I don't see the appeal of dangling things off my body with a sling, especially given the high current where I'm from. I have no interest in sidemounts either.
I've never actually had to use the pony and probably never will, but it is there if I do. It seems like a pretty small tradeoff to me.