In a recent article in The Cochrane Library:
The results provide evidence that reduction to quit provides similar quit rates to abrupt quitting with no evidence that one method is significantly superior to the other in adults trying to quit smoking. This applies whether therapy is self-help or includes behavioural support and whether the quit attempt uses nicotine replacement therapy (Nicotine Patched and so on) or not.
Lindson-Hawley N, Aveyard P, Hughes JR. Reduction versus abrupt cessation in smokers who want to quit. Cochrane Database of Systematic Reviews 2012, Issue 11. Art. No.: CD008033. DOI: 10.1002/14651858.CD008033.pub3.