Are you considering or using Force.com for custom applications? Unfortunately salesforce administrators find that DIFFERENT users need DIFFERENT sets of permissions in DIFFERENT apps. This could become unmanageable depending on the number of unique profiles your organization would need to create.
With Microsoft Dynamics CRM, you can assign multiple profiles to your users to allow them access to the applicable areas of your CRM system and provide administrators flexibility in setting security roles as mentioned in a previous post. If you are considering Force.com and Microsoft Dynamics CRM, check out this whitepaper providing a comparison of the frameworks.