how's the game online
The same as it is in single-player and on LAN. Awesome! There is no distinction between offline play, LAN play or online play. You can use the same characters in all game modes.
For online play Runic only hosts a lobby service and you connect directly to the host. So if you are playing with friends from SA on good connections you should have no issues.
PS. You need to have a Runic account to play over the Internet.