I found this FAQ from another guy who completed P90X and found the information to be very useful.  P90X has been around for years, since 2004, and there is a lot of information available about it.  A lot of information that is probably crap as well.  Like anything on the internet, you have to do a little digging, sometimes, to find the jewels.  I think this is one of them and worth reading through if you are trying to complete your first 90 days of P90X.  In particular I was interested in the Alternative Supplements section.

Another good section is on the nutrition plan.  He mentions how it is confusing for some people and even contradictory.  I would probably agree that it is not as straight forward as it could be.  I think the P90X nutrition guide is trying to provide options and flexibility and this can cause some people to just get lost.  I personally found the Insanity Nutrition Guide to be much easier to understand and have adopted it for now to use with P90X.