Depth or Breadth

Do you consider yourself as a Jack of all trades (aka master of none) or a specialist? If you are a specialist and if you are best in that field, you can command big bucks but then if that field is going downhill, you could be the same too. If you are a jack of all trades, it may be easier for you to adapt but then it may be harder for you to compete against all the specialists. Is it possible to be a jack of all trades and at the same time the very good in all these areas?