What can be said about 2's (AKA Ultiduo?)

So, I overheard a friend talking about 2v2 matches in TF2 and thought he was being a wise guy until I saw some videos online; I want to know more now.

• Is it fun?

• Where can I find a server to try it out?
• Is there a competitive league that supports it?
• What are some things to keep in mind while playing ultiduo?
• Are there certain restrictions in terms of ammo, classes, weapons, and health to ultiduo?
• Is there something really important worth noting about 2s?