Think of Stadia as a free virtual console in and of itself (but one you still need to buy games and controllers for), and you won't be far off the mark. However, obviously, Google wouldn't be able to support such a service without some kind of return on its investment -- not in the long run, anyway.
That's where the Stadia Pro plan comes in. The service will run you $10 a month and give you access to a better, faster overall gaming experience, including 5.1 audio support and 4K. That isn't all, though: Stadia Director of Product Andrey Doronichev dropped another interesting Pro feature tidbit during a Reddit AMA today.
According to Doronichev, the Stadia Pro plan will give you "roughly" one free game a month (to keep, presumably) just for staying subscribed. Doronichev says this is not a "Netflix for Games" approach like some believe, but something more akin to PlayStation Plus or Xbox Live Gold.
The first free game on offer is Destiny 2; which is somewhat amusing given that the game is already set to go free-to-play on September 17.
Doronichev also addressed some other key questions users had about Stadia. He confirmed that the service will not support Bluetooth on launch (but it may at a later date), and users who purchase games will retain access to them even if Stadia is shut down at some point. He did not elaborate on how that process would work, which may be a red flag for some.
Unlike Steam (to name one example), Stadia relies almost entirely on cloud servers to operate. Without those servers, it's unclear how users could possibly access their game libraries.
Perhaps there's a clause in Stadia's publisher/developer contracts that require them to offer users Steam, Origin, Uplay, or other platform-specific keys if Google's ambitious service is discontinued? Only time will tell.
Regardless, if Stadia sounds like your cup of tea, you can pre-order a "Founder's Edition" for $129 to get access to a Founder's badge, a controller, three months of Stadia Pro, a "Buddy Pass" (whatever that is) and a "Stadia Name." Additional controllers will run you $69 each. Stadia as a whole is expected to launch sometime this November.