S.F.
I don't know any one that has tried the Keeper, however, I do know several that use the Diva cup and love it.
I use Instead (Softcup menstrual cups), they are disposable cups sold at Wal-Mart (other places may carry them). I don't think they hold as much as the Diva or Keeper. On my heavy days, I can make it 3 hours without having to change one, where as with tampons, even super plus, I'd be lucky to make it an hour without leaks. The Instead cups are also cheaper than tampons and pads.
The biggest reason I haven't switched is my bathroom isn't set up where I could wash it out and reinsert it, without having to get up and walk to the sink.