London hours offer much better intraday trading opportunities, at least for my setups. Especially if you're focused on sterling-dollar and crosses. In terms of share of the turnover, London is, of course, comfortably ahead of both New York and Tokyo, although that's really not important to most independent traders.
Wood474, maybe we should trade places for a while, hah. Been trading the (brutal) European hours from across the pond for nearly 3 years now.
However, if you're already active during the US hours, i.e., the tail end of the daily forex cycle, you may still find adequate volatility to work with. Obviously, it'll depend on your trading style, time frame and pairs.