I have been using DP4 for several month now. I have used early versions as well, but I have not really used it very thoroughly. David Cardinal who developed it does excellent support for the program. I like many parts of it, but others do not work well in my opinion, for me the keywpording could work better. I does have featuresto keep up with submissions and such and generates good web based templates for submissions. One aspect of the program is that it was developed by people that make a living from editorial and stock submissions. I have been more fully exploring its features of late. I do likethe fact that you can import files from cards and add copywrite and caption information.
There is a forum for it as well: www.nikondigital.org
I hope this helps,