I use 1000mcg of B12 five to seven times a week. Personally I don't feel anything on it (no change in energy or appetite), but I think it does help increase metabolism a little. B12 is very hard to OD on, so you can go higher if you want (2000-3000mcg/day) but I personally find no real benefit.
BTW, I also use it to cut the pain of high BA content injections (yes, even oil based). Works ok for this.
As for B-complex, I have never used it, so I can't comment.
PS: I use an inexpensive vet B-12, 1000mcg/ml of cyanacobalamin. If you want to inject less often, buy Synthelamin from Synthetek. It has hydroxycobalamin and cyanacobalamin, 2,000mcg/ml in a 100ml bottle. The hydroxy is longer acting, so you don't have to inject it ED.
I've been using B12 for over a month now. Going IM. Metabolism is still high and hunger has always been there, maybe increased slightly. I'm taking 1000 mcg every morning. Dr. GoodBody is correct that you can't really OD on b12 and that you can use it to cut some oil pain like Prop or T400. Very affordable just to have around for that situation.
As for the BComplex, you might want to trade that in for just Plain B12. The B6 in the BComplex stings and leaves you knotted up. Kinda defeats the purpose if your using it to cut your oils and can be irritating when you do your ED shots.
I've used it for a cutting agent before as well. Never really ever saw anything noticable physically from it though. It is definately good just to have around for cutting painful gear though! Take care.