Just picked up the Savage Worlds Horror Companion today and I've been reading it over. Figured I'd give a summary of my first impressions.
Chapter 1: Characters
Most of this stuff is pretty good. The Edges and Hindrances are actually really useful for just about any Savage Worlds game. Only a few rely on "special" horror rules like the Sanity rules. I wouldn't suggest buying the book just to get ahold of the extra Edges/Hindrances but it's definitely a bonus that they're so broadly useful.
The biggest problem is the Children of the Night section which provides rules for playing non-human characters (mostly monsters). The biggest problem is that none of these "races" are balanced against one another at all. This would be fine if the book gave you "point values" for the different monsters so you know which can play well together or providing rules for how to balance powerful races against weaker ones.
For example the Angel has a racial point value of at least +20 (there are several abilities traits that are difficult to put values on, it's probably worth more) versus the stand human's value of +2. Dhampyr's on the other hand really have a negative racial value, at least -2. There's no indication of how one is supposed to handle these problems and most of the racial abilities are extremely difficult to evaluate objectively.
Chapter 2: Gear
This chapter contains a fair amount of useful material (such as the effects of wearing garlic strands, waving holy symbols) and some fun toys (such as atomic ghost hunting packs). It has some fairly silly mistakes that shows some people didn't do their research very well. For example a pure silver sword is just as useful as a regular sword (if more expensive) but a sword made from cold-forged iron is fragile and breaks if you roll a 1 on a fighting dice. Silver bullets are also apparently softer and have a lower AP value when silver is actually harder than lead. Silly but nothing horrific.
Chapter 3: Setting Rules
So here we come to the techniques used to adjust SW to a horror setting, the meat of the book.
Some of the rules range from just cheesy to downright ridiculous. For example, one suggests using a cone template to represent the blood spray from a dead victim. Now I know Hollywood gets excessive with the gore...but that's a 50+ foot long spray of blood. I don't remember many movies being quite that bloody.
Sanity is one of the biggest features of the chapter. Anyone familiar with Call of Cthulhu is not going to be at all surprised with how Sanity works here. This will probably determine how you feel about it as well. If you like CoC's insanity system you probably won't have much problems with Savage World's but if you prefer more detailed or realistic sanity rules (like Unknown Armies) then you probably won't be satisfied. It's worth mentioning that Sanity really only seems to be a big problem for low-powered characters. Anyone with a decent Spirit score and perhaps one or two Edges to spare basically never has to worry unless they run face first into a dark god. Even then they'll probably be fine.
This chapter contains some simple, but relatively serviceable ritual magic rules. A quick scan of the ritual rules seems to indicate that they could easily be abused (for example using multiple rituals to layer long-duration defensive and trait-boosting spells) so if you're going to use these rules make sure to determine exactly what's required and what the limits of ritual magic in your campaign will be. As a general set of building blocks for a basic set of ritual magic rules it's not bad though.
Chapter 4: Magick
Th "k" on the end makes it spooky. This chapter is pretty much a big list of powers. The powers here are mostly pretty useful for any sort of fantasy although there's certainly a strong focus on necromancy and other creepified abilities.
Chapter 5: Arcane Items
Like the Magick chapter this is pretty much a big collection of magical gear that could reasonably work in any setting, just with themes of evil/blood/spooky/etc. Nothing's particularly exceptional but it's still a fairly useful collection.
Chapter 6: Creatures
Now THIS is something. We've got 70+ pages of monsters. Demons, dark gods, undead and other. There are also rules for "adjusting" various monsters. Several different "species" of vampires, zombies and slashers and so on. You've got just about every classic movie or literary monster or threat (including nesting alien queens and hockey-masked slashers). There's also a couple of downright silly ones such as the head-banging demonic heavy metal musicians.
Chapter 7: Game Mastering
Honestly I've only skimmed this section and you probably will too. For the most part this is the same advice you can find in just about any RPG book about horror. I'm not saying it shouldn't be here (after all not every GM has much experience running horror-themed games) but it's certainly not something that I feel the need to go over in great detail. Atmosphere, pacing, show-don't-tell and the need to balance subtlety with the gruesome. Etc. Etc.
None of it is is specific to Savage Worlds and I don't think many people buying this will be looking for GM advice but I could be wrong.
The book isn't exactly dripping with pure horror gold...but at the same time it's still an excellent general resource to provide monsters, magic and edges to your own Savage Worlds game. I'm impressed with how generally useful the book is (which is good considering I'm not planning a horror game at the moment) and while it doesn't have much in the way of shining gems there are very few flaws (mainly the issue I mentioned in chapter 1 with the races).
I'd say it's definitely worth the 15 bucks I paid for the pdf.