But since the price per GB is more or less the industry norm, there needs to be (for me) some other sort of competitive advantage.

Reason why I'm on Verizon is solely for network coverage. Since this service is using Sprint and TMob, I don't want to risk not having coverage. Granted, I'm in a major metropolitan area where 95% of my life is gonna be blanketed by all the major carriers, but still. I have some friends that have spotty Sprint coverage during parts of their day. Could Fi's switch to Tmobile cover that? Possibly, but I don't want to leave Verizon entirely to find out.

If they were offering, say, $5 per GB (or something similar for a family plan) then I may consider switching, but as it stands the potential lack of coverage wouldn't be worth a measly $20 or so to me on a $230 family plan.

Some good maps comparing the coverage:

http://www.theverge.com/2015/4/22/8469901/google-fi-coverage-map-comparison-sprint-tmobile

I wouldn't consider the plan personally, but as a business decision I'm curious to see how this goes. Google is known for using negative or neutral cost-benefit endeavors to mine data for other projects, so I wouldn't be surprised if this is another one of those times.