We went to My Kiddy Gym in Causeway Bay for a trial classs this week and my son enjoyed it so much we've signed up.
They only started-up in November, so all the equipment is brand new.
Its also fairly unique (I think) as the 2 lead teachers are male (no criticism of the fabulous female childrens teachers I've met over the years), but it's a refreshing difference for the kids.
Would recommend giving it a try and they are offereing a free trial class anyway!