There has been extensive research into NRT efficacy, but it seems best summed up in the conclusions of a Cochrane Review¹ of 150 trials with over 50,000 participants:

“All of the commercially available forms of NRT (gum, transdermal patch, nasal spray, inhaler and sublingual tablets/lozenges) can help people who make a quit attempt to increase their chances of successfully stopping smoking. NRTs increase the rate of quitting by 50-70 per cent, regardless of setting. The effectiveness of NRT appears to be largely independent of the intensity of additional support provided to the individual.”

Reference

1. Stead LF, Perera R, Bullen C, Mant D, Hartmann-Boyce J, Cahill K, Lancaster T. Nicotine replacement therapy for smoking cessation. Cochrane Database of Systematic Reviews 2012, Issue 11. Art. No.: CD000146. DOI: 10.1002/14651858.CD000146.pub4.