Seemed like a good idea at the time. I was following Shawn Walker with his notion of I Buy Spy, which converted into Dotnetnuke. I convinced my two developer partners to have a look at the framework and they liked it. We built a hosting business around the framework. It was open source and free. We sold the hosting business and I concentrated on my own business. Some of our old customers has a bad experience with the company which bought the hosting business, so I took them on as clients of my business. I am still hosting them today. At the moment we are hosting around 43 domains on DNN.

We don't use it to it's full potential but that's because we don't put enough effort into our own site. We use it as a portal for our clients, they use it as a CMS and some use it just show they have a presence on the web.
I have just been assigned to take ownership of a NPO company website, which I will be looking to convert from WS to DNN.
We still use DNN as our main hosting solution for our clients.

Easy to use and I've worked with it for so long I know it quite well. I also believe it is more secure than other solutions.

100 percent, our clients and our websites have not had any security issues

Yes we are currently upgrading to the latest version

It's easy to use in it's native form. However, there are some really great third party modules and themes available.