Mine's been on it a while and I've not had any issues - I don't think it smells particularly strong so can't imagine they'd really taste or smell it. I feed it to my old pony too and he is pretty fussy and can detect any 'medicine' powders etc and yet he eats it fine.
To be honest i haven't got the faintest idea if it's doing anything as i put my horse on it as a preventative measure more than anything. That was probably about 2 to 3 years ago, he's 20 now. he doesn't though seem to be suffer any stiffness issues. My 25 yo pony is on a maintenance level of the 10:10 as well and we took him to an in hand show the other week - he won his veteran class. My sister said the judge commented on how he didn't show any stiffness at all. is that to do with the Topspec? I don't know! He did have some physio a few years back and the physio said he was stiff. The same physio saw him again i think last year (I just wanted him checking after he'd slipped and fallen when tied up and spooking). The physio said he wasn't stiff at all. I had started him on the Topspec in that time. However, i couldn't categorically say whether the Topspec helps - the physio swears by it and i wouldn't now want to take them off it! It's a tricky one!! Lol