I love that it's easy to access with my smartphone! It makes traveling for work so much easier when you have everything in one place (my phone!) The fact there are different options available depending upon your spending habits really helps too.
We also get an email every month letting us know how we're doing compared previous months - this gives me peace of mind knowing what our goals should be each time around. There isn't anything i dislike about rewards 21 at all honestly :) If nothing else they definitely help keep track of where money goes throughout the year which can sometimes feel overwhelming since most people don't realize exactly WHERE their paycheck gets spent until after taxes come out or something similar...this way lets them do any necessary corrections right away instead of waiting till end of March/April etc..