south coast - Michael O'Leary is not only buying every new FR 738 with winglets but is refitting every 738 he's got with them. MOL doesn't pay for anything he doesn't need - he won't pay for things he should pay for - especially when you consider that the acquistion, refit and downtime probably costs $1m for the in-service frames.
Winglets save more fuel the longer the sector goes, the amount depending on the type of winglet. The tipping point has come now that FR is moving away from the short to the medium hop sector (south of Spain, south of Italy, eastern Europe, Finland etc.) and $62 oil is also a factor. It's not "not proven", it is either appropriate or inappropriate for a given aircraft depending on the usage/sector length. Even on short hops winglets can reduce runway requirements although they may also impact crosswind limits negatively.
Good enough for you?
Have a look on Aviation Partners Boeing's website - they make the "placebos" refits for Hawker XPs, 737s, 757s and probably 763ERs down the road - they discuss the rationale for them there.