What are the pros and cons of using FatFreeCRM?
It appears to be a lightweight, open-source CRM package.
What important features in other CRM packages is it lacking? How difficult is it to migrate from FatFreeCRM to something proprietary, like SalesForce.com or Highrise?
FatFree is a solid system, but there is a tradeoff of having to maintain it yourself. If you're a rails development shop it should be trivial to get yourself up and running, if not I would spend the couple bucks on Highrise and your more important resource (your time) on your business.
Regarding features. I have used everything from highrise to zoho to salesforce and the only features we ever really used were contact tracking and deal tracking. Everything other than that is great at a Fortune 500 company but pretty worthless at a startup. You don't need a report to tell you how many potential customers there are and where in the "pipeline" they are when there are 3 of you in the company ... you know.
Regarding migration. All the data in fat free will be in a database that you own and have control of. Salesforce offers tools to import just about anything so that would simply be a matter of exporting and importing. Highrise, I believe, only allows you to import contacts so you would have to use the API if you wanted to get your deals in as well.
Hope that helps.
I would consider using it if my application was built on Rails. I still like Highrise the best.