Don't add too much casein, or no flavoring will cover that nasty stuff. I have a blend that is about 30% caseins with no flavoring and no sweetener... only a fool would try it more than once.
If you are in doubt, simply order it unflavored and order the various flavor packs for 4 bucks each. you can have a different flavor each time you use it if you like. One flavor pack will handle about 10lbs of protein.