My experience of Fast Track in the UK, limited mainly to Terminal 4 at Heathrow admittedly, is that it's anything but fast and left me wondering how much slower it must have been for other travellers.
My favourite overseas airport with Fast Track has to be Vancouver: when I've used it, there has never been a queue.
And no, I wouldn't pay extra for it. Words like "fools" and "money" come to mind. Bit like paying for First Class on the Heathrow Express.