To be sure you're comparing the same thing - Karma's refuel plan means I pay $10/gigabyte, but it never expires. When it's at half-price special it's $5/gigabyte. So I spent $100 for 20 Gigabytes last August, and I've barely used 2 gig that is still available to me. At the rate I use data it will probably be at least a year or more before I refill, so my monthly cost is trivial, maybe well under $10/month. And no monthly charge.
For someone with moderate-to-low data needs on my phone and laptop and tablet, it makes sense. If I needed more data, like a lot of my friends do, I'd probably do better with some other mobile phone plans
So I wanted to to clarify - do you mean $100/month for the 15 GB, or $100 for 15 GB that doesn't expire?
As for the Neverstop plan - Karma tried it, and the way it was actually put to use turned out to be too expensive for them to maintain. (I never tried that plan btw)