We are currently considering using DotNetNuke as a base for our future portal-based and client-customizable web application that is going to be hosted centrally. The idea is
I have worked on several DNN projects. There are a few things that I always seems to get stuck on.
The first thing and maybe most important is the menus. The built in menuing and the purchased menu modules all were very limited and difficult to use. We used xml transformation to present create the html for the menus. However, the xml that we got returned to us was a flat xml file. Meaning no use of hierarchies, which limited some of the sub-menu things you can do. So Level 0 to Level 4 menu items were all siblings of each other.
Secondly there are in many modules out there to choose from. If standard DNN doesn't do something there is most likely a module for it. However, the quality of these modules varied wildly from module to module.
Last, in the case that you need a module to do something specific and need to build one yourself. It is not that intuitive as to how to do it, and the process changed between the version of DNN we used for two different projects.
I would say that if you are willing to really learn DNN, it can be a really useful tool. But if this is a one off project I think it is better to leave it alone.