There is indeed evidence that mB12 can methylate mercury. However, I also see papers (e.g.
http://www.ncbi.nlm.nih.gov/pubmed/7292517) saying mB12 inhibits the toxic effect of methylmercury, I've seen other papers & reliable sources saying mB12 has a protective effect against mercury.
On methyl-only, I didn't experience symptoms ANYthing like what I saw when I accidentally took ALA. When I first started taking mB12, I experienced gradual improvement of my symptoms. Once I figured out that mB12 was the cause and started using it more consistently, I started seeing more direct cause-and-effect results. With Readisorb liposomal spray mB12 and with Country Life sublingual mB12, I could feel almost immediate (within maybe 1-2 minutes) relief from symptoms. When I woke in the night, very agitated with twitching and tensed muscles, I could pop a CL sublingual and almost immediately feel the tension drain out of my body. Within a few minutes I was asleep.
When I was on methyl-only, I was still having some symptoms. Hard to describe, and it's been several years so my memory isn't fresh, but I remember feeling "not right," achy, fatigued, with trembly muscles. But that was still a BIG improvement over what I experienced before I started taking mB12. The methyl/adenosyl combination seems to work best for me, and keeps most of my symptoms almost entirely under control.
When I unknowingly took ALA for about 5 weeks, my cold-triggered urticaria got MUCH worse, worse than it's ever been. I also started waking in the night with the same old twitching/agitated symptoms I used to have, and B12 didn't help much. Then I started following the ACC group, discovered ALA with amalgams was a Bad Idea, and stopped taking the supplement that contained the ALA. WIthin 48 hours those symptoms vanished.
So FOR ME, as far as I can tell, mB12 and adB12 have been entirely beneficial. I don't experience anything like the symptoms (apparently caused by mercury redistribution??) caused by the ALA.