Got this as a secondary as I have range anxiety. It's now my primary and the 18Ah that came with my D-Class is the backup. I get extra range (I'd say close to 10 extra miles with dual motors running the whole time) with the 21Ah and more, lasting, starting power on a fresh charge than the 18Ah. Combined with my 18Ah (swapping battery as needed) on a test run, I got close to 50 miles before getting bored and went home, with my 18Ah still having at least 50% charge.
It's so awesome not to have to worry about power. I ride for 25 to 35 miles on my 18ah battery. Then when it get low I change to the 21ah battery and I'm heading back whit plenty of power to spare.