I posted something similar in the Women's Only section regarding B6. I noticed that I was losing weight more quickly on B6 than when I didn't take it. I believe some weight loss clinics actually give shots of B vitamins for their patients / clients.
For me this was only temporary, but I managed to lose a good 5 to 10 lb more easily (I had a total of about 20 lb to lose). I still take a B complex for multiple reasons, but one is (as stated on the bottle) it helps metabolize fat and carbs and my doctor has given me the OK. Another is that it helps with my mood so I'm more likely to get up and move. My doc also suggested that the B complex is likely more useful than just one B vitamin.
My theory is that I was deficient in some B vitamins, so my body wasn't performing at it's peak, and once I was back up to where I should be, they didn't help much any more.
I can't provide any science, but I can provide a bit of my own experience. I hope that helps!