You can play proper online custom games and Forge only, for free though. You'll need to invite people from your Xbox Live friends list or find strangers to play with via external means (discord groups, reddit LFG, etc) and add them to your friends list. 343i said they will indeed implement some form of custom game search filter to Halo 5 (both on PC and Xbox One) in an upcoming update, which is the really interesting part, because then you'd be able to find strangers to play against from within the PC version itself.
You have access to any original map and are able to import/export Forge maps from / to the Xbox One Version of Halo 5.
There's no matchmaking, no campaign, no warzone. But you have access to all REQ weapons and vehicles in Forge and custom games (this is also true for the Xbox One version since the latest update, I believe).
It was meant to be a quality-of-life gift to the Halo 5 Forger's scene because of mouse & keyboard support, and is probably also a sneaky way to drive people towards adopting the Win 10 store. Nevertheless, this is the Halo 5 engine on PC with free multiplayer support (you won't need Xbox Live Gold to play online) and a PC custom game scene could emerge from this. As most Halo veterans will hopefully agree with me: custom games are where it's at.
Ah wow thanks for the info man! Super comprehensive! Much appreciated!
This could really do wonders for Halo 5 and its community.