After 7 years at Microsoft I grew a phobia of integration. Integration meant integrating MSN Search w/ Encarta, Office, Messenger, Internet Explorer, etc. It looked great on paper and it was pretty hard to argue it wasn't the right thing to do for the customer and for the business.
The problem of integrating products (and sometimes even features) is that exponential cost of doing anything. There will be miscommunication issues, there will be bugs on both ends that nobody knew before and it's not just a line of code to fix it, and the result is going to be short of the original goal by a lot. Hey, but there is next version where we can fix things...
My core advice to startups is that you should not try to integrate with external partners/services until you really, really understand the cost and benefit and it stacks above everything else you can do alone.
I'm the Co-founder & CTO of 
