Just because its a 20 year single malt doesn't mean its worth a damn. It just means that its not blended.
Blended booze is usually blended for the predictability of the flavor, and while its not as good as the best malt in a series, its better than the worst. A house brand at a place like Costco won't be a super high quality single malt, and it might be on the lower end of the scale. Its up to y'all if you're going to spend $45 on a bottle of this, but you might be happier with a bottle of the blended brands you drink on a more regular basis.
True it could be on the lower end, but my personal opinion after trying one of the two bottles I bought is this on the higher end of single malts.
Am I an expert, no! But I have owned or tried numerous singles near and above this age.
For me it was a SlickDeal.
"When a man steals your wife, the best revenge is to let him keep her"