My experience so far is limited with Local only (Same computer), however as I said it appears to be you can insert your own IP addresses into it allowing you to connect with anyone.
It appears also to be limited to 4 players, although a few of the custom maps I've played (survival) even on that it can get slow with hundreds (or thousands) of enemies. Beyond that it will run really well even with four players.
As for a test example if you want, PM me your IP address (assuming you aren't behind a router or firewall), then you'd go to multi-player, select 'host', then I'd tap into the game from my computer and we go from there.
I'm not sure if it's like StarCraft where if you're missing a map that it will auto download, but I certainly hope so.