Sam,
Saying that DNN is not on par with SharePoint is an understatement. :) Then again, you also looked at the price, and that's a HUGE factor, obviously. Right now my group is using DNN as a Content Management System with pretty good success. Some of the things need to be tweaked and there are always features that you wish were available.
We're using Document Exchange 3 right now. DMX 4 offers webDav functionality, but isn't fully compatible with Windows Vista. Here's a quick rundown of some of the limitations.
With DMX, the file isn't automatically locked when you download for editing. The user has to remember to lock the file they are editing or someone else could decide to edit it for them.
There isn't a good 'workflow' method. I haven't found a way to have person A upload the file and person B is notified it's their turn. There also doesn't seem to be a way to control permissions on a per-user basis. All of the DMX permissions are role controlled.
There isn't an easy way to do a mass download, start clicking files one at a time.
All of that being said, with a little bit of training and attention to detail, there haven't been any major issues regarding using DMX. If the Vista issue gets resolved, or if you can guarantee that Vista won't be used, DMX4 is GREAT, as it allows on-line editing of files (only tried MS Word). I'm not sure to what level you would be using sharepoint, so it's hard to point out what other features you'd need to look at. Hopefully this was helpful.
Rob