i use'em with clients. you can train soccer moms and fat hockey dads with 2 sets of kettle bells 12 and 16kg, a yoga mat, a 3/4" band, a 2" rope, and a TRX strap all day. throw in some thai pads and sparing gloves maybe and a 30lb solid lead bar.
I'll use kettle bells myself for 1 arm overhead tricep extensions with a 24 or 30kg. easier on my wrists than a regular dumbell. that's about the only bodybuilding use for them.