My organization has been using Microsoft Dynamics CRM and AX for almost 2 years now and have gone through one cycle of upgrades for both in that short time. And the reason? well, sad to say, is because both products fall short of what we wanted (and promised by vendor) initially as well as even now. Below are just a few examples of our disappointment:
1) AX Multi-timezone issue - we adopted the solution as a regional platform supporting 12 countries spanning 5 different timezones. Amazingly, the so-called enterprise class AX (previous version 4.0) cannot do this OOTB and even our vendor's promised workaround posed serious limitation to the way we do things. Having said that, the latest version 2009 does solve this problem.
So in conclusion, IMHO, Microsoft Dynamics (especially CRM) is probably more suitable for small local shops rather than regional platforms with large number of users spread over geographically diverse locations.