Generally B12 is a good thing. Some people need to start at a very low dose to avoid possible side effects. My guess is these people may generally be quite sensitive to many supplements or medications.
According to Drs De Meirleir and Cheney and other researchers the most effective way is to inject the Hydroxocobalamin form under the skin. Under the tongue forms of hydroxo or methyl are also found to be effective by some people. The cyanocobalamin form isn't recommended by any doctor I've heard of for ME/CFS and Cheney once said it should never be used. Though just to confuse me one patient blogger has found injecting this form helpful and better than the others!
I understand B12 is a good scavenger of nitric oxide which can be a problem causing brain fog. (This is from Dr Pall's research.) I've found Hydroxo B12 very effective for this and have much improved concentration since injecting several times per week, now daily for several years. For me, injecting 0.07ml of 10mg/ml strength daily is very helpful. Some need far less than that to start, others much more to get a result.
Probably the only way to know is to try it yourself, start slowly and see if it helps.
Folate in the form of methlyfolate (metafolin) at 200-250mcgs per day is an aid to helping the B12 work according to my docs.