I'm kinda fuzzy on the whole alliance thing, but what I do know is that forming an alliance costs 1 billion ISK plus 1 million ISK per corp in the alliance per month maintenance.
So it will prolly be a while before an official alliance is economicly feasable.
The benifits I know of are an alliance channel like the corp channel (This however is an advantage that can be created without an official alliance simply by creating a chat channel for the people in the unofficial alliance) BTW I think we should prolly have a common chat channel for Avian & TCO as soon as someone figures out how to make one.
I honestly don't know of other benifits (But I know there must be many since hundreds of corporations have invested the money to become allied)
The disadvantage of an alliance is that you are inflicted with your allies baggage. If someone declares war on an ally, you and all your allies are automaticly at war also. I know Avian's policy on War and TCO-E has a simular policy. (Well our policy on war is not cast in stone but basicly states that we walk away when we can and if we can't then we run.)
Also from what I understand alliances are mostly for corporations in low sec space. And with the exception of ISS which has managed to remain neutral, corporations in low sec space maintain control over star systems thru force and frequently engage in conflicts to maintain their territory. ISS has however shown that neutrality in deep space is possible if you do things right.
-LA