There isn't much to it really.
Go into the multiplayer menu.
(pro-tip, the master authorization server was taken down, so add the following two entries into your hosts file "/windows/system32/drivers/etc/hosts"
and instead of waiting 2 minutes for the time out, it will time out instantly)
If you are hosting, New game (or load game for subsequent play throughs) just set up your server once, and it keeps the settings from then on.
To join, just use the direct connect button, and type in the IP directly. No virtual LAN required.
Performance is fine, I've rarely noticed any lag issues. Occasional rubberbanding, but it's pretty rare.
I've not personally done any modding, so I can't really speak to that part.
As far as the original campaigns go, they work pretty well multiplayer. I recommend turning up the difficulty, or taking less companions though, it doesn't count PC's against the party limit, so you could take a large party through the game and all the companions as well. In the Original Campaign, if you have one player as the party face, they will want to lead the party, because the scripted scenes tend to choose either the party leader or the closest player to do the discussions. Only the person actually talking gets to use Diplomacy, Wisdom/Int conversation options, etc. In Mask of the Betrayer and Storm of Zehir it seems to only be the party leader that does the talking in those situations, so you can put the tank out front.