I'm a bit skeptical about their effectiveness. I'm an engineer although not in aeronautics so I don't know any technical info. However I remember seeing a press release a couple years ago where Westjet said that the winglets improve effeciency by "up to 4%" and later in the release said "statistically 90% accurate". At the time I thought it was strange that they relaxed the normal engineering statistical tolerance of 95% and qualified the 4% with an 'up to'. Also, small number statistics that happen to be just under 5% difference are questionable too for the same reason. I'd like to see the statistical distributions used and data and parameters (how many flights, how did they measure the fuel burn, how to control the experiment), which I know won't happen. There's always a given mean and standard deviation for a data set, and a regression analysis of fuel efficiency would have to account for dozens of factors. I'm not saying they didn't do it though, but that small number statistics are difficult in the best of cases.
It's also always struck me as a bit strange that they're only coming out with these now. See how similar the new airliners coming out are (A380 and 7E7), large scale advances are behind us (not aeronautical advances though). Think of the Concorde and SR
-71. It's true, that todays technology allows us to identify those last couple of percentage points that yesterdays tech couldn't. So I'm not saying aeronautical engineers don't do anything these days, they actually more interesting jobs. Instead of working with aerodynamics and wind tunnels they work with simulation and models. In fact, they do a lot of software programming, imagine the challenge of making the software for the stealth bomber to fly like a regular airplane.
Anyway, my point in this ramble is that they probably have an effect, but it might be smaller than advertised, as a 1 to 2% efficiency difference would be hard for airliners to verify, and they might not be worth it in terms of cost/benefit. Maybe someone can provide some links that aren't just press releases. But they look cool, and are advertised to save fuel, so why not?