Wonderful

I've been using reusable cups for about 6 years now.  I initially bought the Keeper, misplaced it one month, and bought the Diva.  Which I have also misplaced, but it's ok because I found my Keeper back.

I think people's questions fall under two categories- what are these cups like in general, and then which cup is the best.

In general-

The cups are fantastic.  They last for about 10 years, and only cost around $30.  Compare that to other products, and you'll see the money piling up in front of you.  I also love that I can stick it in and forget about it for 12 hours.  Again, no other products can compete there.

Then there's the environmental benefits- less production required, less waste generated, no bleaching used...

Health benefits- no association with TSS, doesn't dry you out like tampons can, no exposure to dioxins...

The only problems I found were that it's a little tricky the first couple times you use the cups, and if you are having a heavy enough flow that you need to empty the cup during the day, it's a little awkward in a public bathroom.  I like to use soap and water to wash mine in between uses, but I suppose you could carry some wet wipes so you wouldn't need to leave the stall with your cup.

Diva vs. Keeper-

The Keeper is made of gum rubber, the Diva cup is silicone.   The Keeper is not good if you have latex allergies.

The cups vary a bit in shape and stem length.  The Keeper has a longer stem.  When I first bought it, I found this uncomfortable.  I cut a bit off, and haven't had problems since.  This link has a visual comparison chart of the different brands- just scroll about halfway down.

Since I've never had the two cups at the same time, I can't tell you which I actually prefer...