I have done two different Subscription Boxes: Loot Crate and Short PAR 4
Lootcrate I just did their basic nerd box full of nerd stuff. I really did enjoy it but when my year came up, I was in the process of trying to buy a house so I didn't want to re-subscribe and have that money drain like that. At the time, it was absolutely worth $12 a month or whatever it was because you got some pretty cool things with it usually (and if you didn't, it wasn't like you were gambling a ton of money). I have heard that they went through a bankruptcy or someone bought them out or something like that so I have no idea what the quality is now.
Short PAR 4 was a little different (it's a Subscription Box for Golfers so you would get Golf Polos, Shorts, Pants, Towels, Shoes, etc). I actually had really liked SP4 but the biggest issue was the price. At $50, getting something you didn't like hurt a LOT more. While a lot of your clothing were some really nice brands, they also mixed in their SP4 Brand a lot (and it seemed to happen more and more as the subscription went on). The SP4 branded stuff wasn't bad but there was a clear quality difference between that and something like PUMA or ADIDAS or NIKE. It sucks because most everything you're getting would likely cost you $80-100 if you were to buy it in-store but getting the SP4-branded stuff three months in a row left a bit of a sour taste in my mouth so I left.
In both cases, it comes down to your Risk (how much you're paying) versus your Reward (how good of stuff you're getting and will you use it). If you're confident what you're getting is good and/or useful, subscription boxes are an awesome way to get new stuff at a discount (and if you're like me, you actively enjoy the surprise of seeing what random goodies you got).