With the current version (and all previous) Composer will get an error when trying to register content areas on a PageTypeBuilder page, this is due to the way Composer loads the content areas. With just four lines of code this can be fixed and it would be great if this could be done in the main branch instead of having to have a custom PageTypeBuilder. As far as I can see the changes should not interfere with sites that don't have PageTypeBuilder. The changes needed and reasons behind them can be found in this blog post, http://labs.dropit.se/blogs/post/2009/10/22/How-to-get-PageTypeBuilder-to-work-with-EPiServer-Composer.aspx. It's an old blog post but I have implemented this changes in the latest version of PageTypeBuilder and they still solve the problem.