I have them, and maybe it is being too hard to say "don't prefer them": they are fun for walking around in and I don't regret getting them at all. For actual beneficial exercise, though, I prefer barbell-type exercisors.
What if your vaginal muscles are already pretty strong and tight? Can kegel balls actually make you more "loose" if you start using them? (Can they actually stretch the muscles a little bit, thereby making them weaker?)