Unfortunately, while they feel great, they have on average about an 82% effective rate against STDs and pregnancies.
One of the other issues is disposing of them as they are too large to just flush. I found a great way that works for me to get rid of them, is to use one of the free coffee cups and lids that normally are next to the coffee machines in rooms, place them in there, then just drop them into a garbage receptacle on the maids cart.
They can be found at CVS and Walgreens as well and run about 18 dollars for 3.