limewave, are you sure is a 20/47? it seems like a very odd number, most double cranks ( what you call compact) start at 24 for the small ring and go up to 44 for the big ring. A size 47 will be too big on a mountain bike. A 20 teeth granny gear would be super easy (too easy) and a 47 would be too hard. Using a double really depends on the area you ride and you level of fitness. Here in SoCal most of the fast racers are using doubles and really like them, but you must consider that they don't use the granny gear at all so having it make no sense and they tend to ride bigger gears anyways.