I think it has changed a little since I originally signed up...I believe the discount is best as a Prime/Mom member doing the subscribe and save feature, which means your diapers ship out every month (or every other or however you set it up), and you receive them toward the end of the month. You get the full 20% off this way, it's broken down as like 15% as a Prime/Mom member and 5% for the Subscribe and Save option. The shipping on S&S takes longer so if you need a last minute refill you get the free 2-day shipping but not the extra 5% off.
I love it and use it for my diapers, wipes, Diaper Genie refills, diaper cream, etc. etc. etc. You can always cancel orders before they're shipped so say you got Desitin delivered monthly for three months and now have extra, you can just skip the next month.
Plus a free Kindle rental book each month from their library and lots of free TV shows and movies as part of their free unlimited video streaming, and free 2-day shipping on EVERYTHING.
So worth it. You will start using it for EVERYTHING. Diapers.com might be a little cheaper for just baby stuff but when you start using it for other stuff too you save money and it's so damn convenient to get the free 2-day shipping. I used it for stuff like planning DS's birthday party (buying decorations and goodie-box stuff etc) as well as Christmas and birthday shopping. Free returns too.