I like the idea. It's great to see this race to the bottom on data pricing!
Think it's useful for:
1. Sharing wi-fi access with others without worrying public users will inadvertently use up your data allotment or cause you to pay data overages. Plus this service actually encourages and rewards you for sharing
2. Commercial users. Might be ideal for a small business to set up a wi-fi hotspot for customers with minimal equipment outlay.
3. Those on very tight budgets. Can see a shoestring traveler or student using one of those to get portable wifi access gratis by having others access the public portion of the hotspot.
Might consider one of these myself if they expand the NYC test area to CT. Realistically my primary access is by AT&T postpaid LTE modem with a FAN discount....that gives me coverage in rural areas(including home that is an AT&T and Verizon area, but has no Clear coverage), at work and when traveling. The Netzero Clear modem I'm using right now is for secondary service(200 mb free plan), as is the occasional free wifi hotspot I run into...these help stretch out my 5 gb AT&T data allotment.
I'd probably use Karma as a courtesy when traveling so others who don't have cellular modems can get on the internet for a 100 mb session for free...the bonus data session I would get would be a nice perk.