I've now used WPMU support a number of times and have nothing but positive things to say. Every supporter digs into my specific issue rather than referring me to some generic help article. Multiple times I've been provided with very specific, individual code to use for my issues (sometimes not related to WPMU at all). One particular supporter (Tho Bui) has even written a mini plugin to address an issue with another WPMU plugin that was caused by our own unique setup, rather than the plugin itself. All in all, WPMU is so far absolutely living up to their support promise!