The plans and availability are fairly basic but limited. First you need to sign-up for an invite and be accepted. Second, the new service currently only works on the Nexus 6, which you can buy directly from Google for $649-699. Google states the Nexus 6 because "It works with the Project Fi SIM card, which supports multiple cellular networks, and a state-of-the-art cellular radio tuned to work across network types."
Since Google has partnered with Sprint and T-Mobile coverage appears to be quiet good, for those wondering Google has an interactive coverage map here. The up-front pricing on their plans seems pretty standard. A "Fi Basics" plan, will run you $20 a month for unlimited talk and texting, plus taxes. Data is an additional $10 per gigabyte a month.
One very unique aspect of Project Fi billing is that you "never pay for unused data." Your account gets credited, in money, for data you don't use. The example shows an unused 0.6GB of data gets you $6 back, so credits aren't limited to 1GB increments; overages work the same way, with no extra fees.
T-Mobile CEO John Legere, who has a long history of supporting the shake-up of the mobile industry, wrote a blog post praising Google's new service and stating that, "Project Fi is going to make people think differently about wireless—and [he] loves that."
Source: Project Fi and for more details checkout Google's official announcement