I use them everywhere.
I don't often use generic definitions in docblocks today, but when I do, it's a mess. I think this RFC is the right move at the right time, and cost (both runtime and syntax) appears zero to minimal at best. It'd be really great to see generics land in PHP. It's been a long time coming.
Generics are one of the most requested feature since PHP met the OOP paradigm and for me, this will step to set the boundaries to its OOP complete adherence.
We will be able to describe complex classes or dynamically typed ones, increase readability and move type hinting away from comments.
I've been writing about this for years. If we ever want generics in PHP, this is the way.