I personally like flats better because they give your feet more freedom. If you’re carrying speed through a turn you can easily let your foot down for a second to get that much needed extra lean. Although I am quite comfortable with clipless, when I ride flats it just gives me extra bailing confidence if I need to get off the bike and fast. The big downside though is your climbing abilities greatly suffer when you use flats over clipless. One of these days I probably convert to clipless as my bike is pretty heavy.
I agree completely with what "Cavermatthew" wrote above…..the abilitity to bail or to use my foot in a moment of need has saved me numerous times (I also like that there is no chance of the pedal not releasing when you take a bad spill, and thus having the bike bang around you or wrench your ankle)…..however I also have found that one can get used to climbing well with platforms, you just need to learn to be smooth in your stroke and learn to pedal "circles" in a slightly larger gear, to compensate for the lack of being able to pull up. Way back when I did XC I used to do only clipless…but when I started doing technical trails, DH, Enduro, and Jumping I went platforms on all my bikes. My current platforms weigh pretty much the same as my shimano XT and XTR clipless did, if weight is a concern.
In the end it really comes down your riding style, the type of riding and trails you do, and what makes you feel the most comfortable and confident in those conditions.