Pros: Saves $$$$, saves environment, 12 hours between changes
Cons: A little tricky the first time, can be awkward in public bathrooms
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.
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...