I know valcyte definately helped me and when i came off i crashed within 8 weeks. The quick fall has shown me how much it helped as many symptoms have returned or intensified since i stopped. I think for those who respond, the issue maybe how to maintain this improvement when off valcyte. I think this is where they need to look into ways of improving immune function.
My impression from reading the studies of the clinicians' records is that for many people, the improvement maintains in the long term. It's a while since I've read the studies so my memory of that could be wrong. As with so many things with ME, I wonder if there's a range of responses from zero to temporary to almost permanent.