Its a bit of hype for sure...how to introduce advances in wing design, without telling everyone the previous design wasnt all that good!
Notice the 787, no winglets there....they use a raked wingtip design, which I believe is what the 777x and A350 will all have..
With the wing, the airflow at the leading edge is at the same velocity across the wing. The trailing edge, as the wing section gets smaller, the laminar flow begins to separate from the wing skin surface, causing turbulence, or cavitation, drag...much the same as in all of the oversimplified stall sections we have seen.
The addition of the winglet reduces this turbulence in several ways. The one most likely to have the greatest benefit is the decreased angle of attack of the wing tip section.
This is fairly easy to correlate with data out there. From the charts, it shows that winglets have little difference in takeoff, and only slight increase when you get to heavy loading at higher altitude airports.
In cruise, when heavy, the winglets provide the most benefit. As the required angle of attack decreases with reduction in fuel load, the benefit of the winglet decreases.
If you are always running light, or short flights, you will likely see little benefit of the winglets...