Happy to provide my opinion, a lot of sites offer "mystery" deals these days, if you have any specific site you question the legitimacy of please ask us here to get feedback from anyone that's purchased from them before.
In terms of the quality of the mystery wine packs where they say you're saving 74% of the RRP - most of the time yeah, these are relatively inflated prices however in my experience, if i'm paying 15$ for a bottle that was advertised for RRP 50$, it's usually tasted what i would expect to be closer to a 20-25$ bottle.
There are some brands out there like for example the 1847 / grand pappy brand who have RRP's of 320? 480? 450? 350? Cant recall exactly, but around those ranges for their "top end" and are frequently discounted to 49$ a bottle packs, or 59$ a bottle packs, people that have tasted those bottles have usually mentioned they're rather good, perhaps around the 60-80$ range, but no where near the 300/400 range.
So if that helps a bit, the general consensus i'd say is that you do get some value in terms of overall quality usually. However the most important thing is individual tastes, if you're partial to specific styles or varietals, then getting a blind pack might give you only 3-4 bottles out of the 12 that you actually enjoyed as opposed to perhaps just getting a 6pack of a well reviewed shiraz of the season that everyone's going crazy for (for example recently there was a powell&sons/Neldner road shiraz that was on winestar for i think 20$ or 25$ a bottle - but in a 12pack? and consensus upon buyers was that it was very good value.)