I'm running a homebrew Underdark campaign mostly using Out of the Abyss as a guide. I was excited to get this product as it stated that these encounters could be "added as random encounters in your Out of the Abyss/Rage of Demons campaign", great!
I was expecting some new and different things that I could throw at my players but after reading it now it's clear that there is little different here at all from what you'll find in OotA. For at least 7 of the 10 encounters my party have already encountered, or have plans to encoutner, something extremely similar if not exactly the same. Out of the other 3 I feel only 1 is worth throwing at my players. Frankly I expected a lot more than just ten pages of monster stats that I could find in the Monster Manual and 1 paragraph below them of setting up the encounter.
So please be warned that if you own OotA and the MM you may feel this is not worth it.