I use Tag for Genres, my own, I have simple needs. Seeing as I have more than one person doing Rating, I have to use a Text column of numbers 0 to 5, because that is the only way to get it into the Tag Browser.
With the growing popularity of ereaders more and more people will be using calibre with multiple users in one database of books. What would be nice is the ability to put custom ratings and yes/no columns, in the Tag Browser.
Or maybe even a User facility, where you can select between users, each with their own set of columns. That would be the ultimate solution.