Just come back from Milan, where today I've passed my 2nd Microsoft Business Solutions Certified Professional exam, Navision 4.0 Development I (formerly known as Navision Programming).
My choice was to take the Development II exam first and so today I can say that the Development I is more easy than the Dev. II. Development I is focused on pure Navision coding, so if you have a good experience on programming with Navision I think you're ready to go (for me the most tricky parts are the basic standard convention, XMLPort and Automation).
However, I feel good today... 
Maybe I've done the 2nd pass for the Master certification? 