From what I have seen with it, the Y/D is like pretty much any other ‘medicine’ out there, works great for some and not at all for others. I would highly reccomend it as an option, along with I-131 and Tapizole, but it may well be a trial and error to see what works for each cat.
One thing to keep in mind is that Tapizole and Y/D both treat the disease, while I-131 cures it. For some that is enough of a reason to fo the I-131 route, others don’t care if it’s treated or cured as long as the cat is doing well.