I second Land After on that point. If you work out the 'pounds saved per airmile' cost of flights you get somthing like this (all pre-tax):
Paris Economy = £60
Airmiles = 450
Value per airmile = 13 pence
Paris Business = £270
Airmiles = 900
Value per airmile = 30 pence
New York Economy = £220
Airmiles = 2900
Value per airmile = 7.5 pence
New York Business = £2000
Airmiles = 5800
Value per airmile = 34 pence
New York First = £3500
Airmiles = 8700
Value per airmile = 40 pence
So the moral: use airmiles to go in FIRST! (or if you don't have enough, go in economy and come back in First for 5800 miles - no problem with jetlag then!)