Yeah, I take all my mestinone dosages (6 - 8 times a day) with a cuppa joe. I've started drinking it when I was 4, so my body is very adjusted to it.
I also take my prednisone with coffee

(yes I only drink coffee and herbaltea).
It does work for me, I've tried many times what happens if I take less. The difference -for me- is significant, I guess it works a bit better than mestinone. Not comparable with prednisone and such, of course, but my "100 mg mestinone - 2 coffee - 1 banana" is my regime if I want to feel a bit stronger.
And, plusside, it's yummy! So that means happy = good for MG!
But I don't think you should overdo it, caffeine can make you stressy,which is baaaaaad for MG.