It depends. I had a Valhrona 33% the other day and it was the best milk chocolate I've ever had. I usually hate milk chocolate. In general, for darks, never below 70% unless it's a chocolatier I like. For example, Madecasse makes a 60-something percenter that I HAVE to try. Delicious stuff.
Everyone here should try Rogue Chocolate. Seriously one of the best culinary experiences I've ever had. One man operation too.