My opinion of the 15 yr sherry bottle is good, but not great enough for 49.99 that I would replace it as one of my fav. sherry bottles. I'd rather spend an extra $10 for something like auchentoshan triple wood, much better.(though it may not be fair to compare ) Don't get me wrong , I enjoyed the 15 yr Kirkland, and glad I tried it. Those are my thoughts considering I'm in CA, where we have great prices & options for scotch.
I got the 20 yr bottle, but it'll be awhile before I'll get to it. I usually stick with the 12-16 years. This would be my first trying a 20 yr old bottle, so not sure how to compare. I think it's definitely worth a shot for that price / age. What some people dislike, others may love.