The numbers are close to accurate for my slower-selling titles. But, yeah, it really underestimates on anything selling well.
Still, it's convenient to have Amazon links to all my books with most of the different Amazon sites (.uk, .jp, etc.) all in one place. I check it daily. (And then don't really DO anything with the information, but... I check!)
ETA: I also use it to check on other authors - not necessarily in a competitive way, but if someone's offering lots of advice/proclamations on publishing or book promotion, I'll type their name into Novel Rank and get a rough estimate of where their sales are. Higher than mine? I should consider the advice. Lower than mine? I'll still consider it, but only on its own merit, not as something from 'an authority'.