COTS vs. Custom Development

This is going to be a debate that will continue for the foreseeable future and one that I'm faced with right now. I'm currently on a project where the customer is are going with custom development. I do think this is the right decision. The main reason is that the company has enough unique business rules that don't fit in well with what a Commercial Off-the-Shelf package can offer.

I can't say that everyone at the company agrees with my opinion. So who is right and who is wrong. How can you determine what the right solution is? How much time should be spent coming to the conclusion? So many questions, but not alot of resources availabe to answer them. Is this something that is gained through experience?

My past experience tells me that even going with a COTS, if you have enough unique business rules your going to have to customize the solution anyway. This can of course effect any future uprades to the COTS solution. So your essentially blocking your upgrade path. Wouldn't it make sense to develop a custom solution at that point?

Isn't software supposed to make your life easier?