DNN is pretty good as far as open source CMS. Plone, Rainbow, and Joomla are also not bad though.
The only complaint I've ever hear about DNN, is that it's infact, overkill on some projects. With that said, I do like how in DNN 4.x, the host can choose which modules to install. That's a nice add-on, I must say.
The best modules are, in order, the following: Blogs, Chat (3rd party), Forums, Store/E-Commerce (again, some 3rd party ones are better than the default DNN comes with), Wiki, and Feed Aggregator.
Does anyone know where to get a good spell checker?