Clipped in is definately the most efficient way to go for xc purposes. I love Time pedals. good all weather (even snow and ice) and durable. I hate Shimano in general as a rule. (I can't help it, they are like microsoft, or Martha Stewart).
1/2 the time I ride flats though. On my #1 bike, which is my SS, I normally ride flats. I love to hop on it and go for a rip at a moments notice, without worrying about what shoes I am wearing. I don't mind doing long rides on them. And flats come in a wide variety of cool colours and shapes. Pedals are my favorite bike jewelery. you just have to look to the BMX world for variety. Us MTBers are a boring bunch when it comes to colour.