I was in Chemo one day and the guy in the chair next to me was getting IVIg for CIDP (going on six years so far). His symptoms sounded a lot like yours, though more symmetrical, and it might have been numbness rather than weakness. The IVIg has an immediate positive effect for him; he says he maintains at about 80% of normality.

I've been diagnosed with MMN by the Mayo. I do not know if the IVIg is helping me--the only way to tell would be to stop and see if symptoms worsen more rapidly. Luckily my insurance, Blue Cross, has not balked. Yet.

I started with weakness, tremors, and fasciculations in my left hand five years ago. Last year I started losing extensibility in the fingers, one at a time. I can now only extend the middle finger, which is useful in road rage situations. I now have fairly constant fasciculations all over, muscles cramp easily, and my right hand is getting weak.

MMN is so rare that it's hard to find similarly-afflicted people to share experiences with. This forum is the best I've found so far, but activity is sparse. I'm planning to start an MMN blog ** soon to make that easier.