This is very interesting. It is a serialization format generalized to handle real references, including circular ones, and more complex data-types than JSON can handle. I think it's a very poor tagline, since "eval in reverse" sounds useless, and this lib is anything but. (no affliation btw)

The down-side is that a consumer has to run a full-blown eval() (as opposed to the more restricted JSON.parse()). This isn't that much of a downside in a typical webapp since you have full control over the browser process anyway, but it's deadly for cross-domain.

The upside is considerable for certain data-structures that are hard to represent in JSON efficiently, e.g. with a lot of denormalization.

A key concern for me is runtime efficiency, particularly compared to JSON.stringify.