I am about to run some updated 2nd Edition content, and I wish I had purchased this sooner. It is saving me a ton of serious headache in converting the numerous monsters not found in the Monster Manual, Volo's, etc. While I haven't playtested it yet, this bestiary appears to have solid conversions of many classic Ravenloft monsters (not a small feat despite assurances that 2e to 5e conversions should be "simple").
My gripes are only about this book's polish. The layout can be confusing, with monster descriptions being separarted from the appropriate stat block by another monster's stat block. I also really wish this bestiary included some art. I am sure pulling in the old artwork is impossible, but some original art or artwork similar to the original TSR depictions would really make this a terrific offering. I do hope that the author considers a 2.0 update that takes it to the next level. I'd happily pay again for it.
So I highly recommend purchasing this if you plan to spend time in the demiplane of dread or a similar horror setting. This bestiary is indispensible if you plan to go revisit the 2e trove of Ravenloft content.