Without question, this site is your goto for all things wordpress. As someone who is developing in WordPress, you certainly cannot "know it all" and hence you need to turn to someone to help solve or help guide you to customize your needs. WPMU is just your site.
Their forum and search library is extensive and they are very responsive when you have to open a ticket for a specific issue. Also, they offer live chat support at various times to support the global workforce.
All n all a great value, to help me when I run into an issue with WordPress- and it doesn't even have to be their products- though I elected to use several plugins because of their guaranteed integration.
Very satisfied member.
Mike P
Polaris Group LLC