Depends on what you are targetting... I've been involved in a couple of projects (as an architect on parts of funcitonality, and as consultant on different parts of implementation), and I can say that it can be done... it looks nice, and can work quite well. But as I said... case by case level.

As for the login part... if you set up your DNN env. as windows auth. enabled it will work great, the user won't even see the difference. The problems come when web (dnn) and intranet (sharepoint) are not on the same server, and dnn cannot be on win auth. Then you need have replicated DNN installs and other things... all in all, it can be done, but not without effort... everything depends on your scenario.