I think mutt has it about right. The further you fly, the more effective winglets become as they really only reduce fuel burn in the cruise (and can help hot n high take-off performance). If you were flying trans-continental U.S. for example, you'd get a good saving (6% is probably on the high side - 4.5% is a bit more like it in reality over about 3000nm).
As Ryanair fly relatively short sectors, the weight penalty doesn't outweigh the aero. benefit by much (really short sectors can actually show a penalty for winglets due to the weight) due to the relatively high amount of climb and descent expressed as a percentage of the overall block time/distance (thus taking account of taxi times). Plus it would add about 3/4 million U.S. to the price of each airplane! On the other side, they do look sexy!