I have a couple of clients who use DNN as their CMS and they have been very pleased. Both are national associations One uses IMIS as their AMS, but there is currently no integration with IMIS. This has been postponed to a later phase as they work to move their web properties from a custom ASP.NET solution to DNN. The first web property to be moved was a microsite. Based on the success of this migration, the association is in the process of moving their remaing 5 or 6 other web properties to DNN.
The second client has been using DNN for about 4 years now and loves it. They have some basic integration to allow DNN login data to be udpated from their backoffice data, which is Avectra's old Abaze AMS. They are in the process now of moving to Protech's Microsoft CMS-based AMS called CRM for Members and they plan to continue using DNN for their CMS.
On the AMS side, I've worked for years now with Avectra clients and their netFORUM product has really matured into a solid AMS. They also have versions available for organizations of all sizes.
Regarding SharePoint, it works well as a collaboratoin platform, but it's not the best WCM solution for most organizations. We just recently completed a web migration using MOSS 2007 and found the process to be much more complicated and restrictive than working with DNN. If workflow and versioning are important features, then another WCM like SiteCore or Ektron would be better options than SharePoint. Of course, they're more expensive too, and many organizations either don't use these features or they could get by with some of the DNN modules available which provide basic workflow and versioning. Another thing to consider at this point is that both workflow and versioning are on the horizon with DNN.
One other thing most associations need to consider with DNN is the need to replace the built in search with a more robust search solution. Again, there are some great modules as well as custom solutions available which help to bridge this gap. All-in-all, DNN makes a lot of sense for associations to use as a WCM framework, especially if the staff of the organization or a trusted vendor have experience with DNN.
Feel free to give me a call. My company specializes in working with nonprofits and associations and I would be happy to talk with you about your particular needs.