If you are only going to use a few pages from the demo templates, I would recommend setting up a demo site and manually export the pages you need.
Then copy the aspx/cs files you need to your solution. If you are using resharper, it is super-simple to move your code to a different namespace. (If you do not have Resharper, it's not that big a job...)
My experience with using the full demotemplate package, is that you end up with a lot of page templates and code files that you never use. That's why I recommend doing it manually.
I have seen the forum in the demo templates and would like to reuse these in a forthcoming site. Has anyone got any tips for moving the demo forum in to a new project. I am reluctant to install the demo templates using Deployment Centre as the last time I tried this I had probelms with conflicting namespaces and it took quite a while to unpick it all.
At the moment I am considering manually moving the files in to a new project and manually creating the required page templates within EPiServer. Any one got any better ideas?