As I hit level 40 last week I have access to the buddy system early in GO. :B
The system seems alright. You throw berries to feed them, keep feeding every so often to see them on overworld. You can increase their buddy-ness, grants perks (catch assist, find presents; find locations (?) and find souvenirs (guess bonus items specific to this); cp boost (I am guessing effectively 5 levels) and a ribbon.
There's much more stats for the pokemon set as your buddy, and daily activities like battling, taking a picture, going somewhere new with it...
Ferroseed rolls around you, it's cute.