[Exim] Exim 4 - How much pain?

I have a reasonably comples exim.config file and a very complex filter
file running Exim 3. For those of you who have a similar setup - I was
wondering about how much pain is involved in upgrading to Exim 4.

Is this a major serious painfull upgrade - or does the converter program
actually work pretty good. What I'm wondering is - is this a long
painful process or a pretty much just works process?