I take injectable B12 pretty frequently.
I don't notice much while taking it (I get used to it), but I know it is good because every time I run out, I notice feeling a little less energetic once I haven't used it for a week or two.
A lot of doctors say that most people are deficient in B vitamins, they are water soluble, so you can't overdo them. I always suggest people take B vitamin supplements, they are cheap and can't hurt you.