The title is a bit misleading, as this book really only collects Marvel's Son of Satan & Satana stories, rather than a cumulative oeuvre of the entire Marvel horror canon (I was especially disappointed to find it only contains the Santana stories in the Vampire Tales and House of Horror magazines included. When's there gonna be an Essential Gabriel the Devil Hunter ?) Anyway, tonally the SOS & Satana stories are pretty different (SOS is more a traditional comic, whereas Satana is more sleazy mature European comic) so I'll talk about them separately.
I was only tangentially aware of Son of Satan before this, him being a pretty fringe character in the Marvel universe. Let me tell you, I wasn't missing anything. Right from the getgo, he's somebody Marvel didn't really know what to do with. He has all the trappings of a superhero (confusing transformation, incredibly silly and unnecessary costume, stupid super powers, a mansion that would make Bruce Wayne blush) but I have to imagine he's for older readers, because what God-and Charles-Manson-fearing parent would let their child buy a comic book where Beelzebub appears on almost every cover? I mean I know this came out before the great Satanic Panic and everything, but 'cmon. (BTW, I think it's a real artistic mistake to show Ol' Scratch; he just comes off as a grumpy old dad, and it would've been better to leave him to the reader's imagination). At any rate, Son of Satan's tormented battle between his good side and evil side would likely be too confusing for a younger audience (it is for me, at any rate) and it usually just exhibits itself with him turning into a huge jerk and slapping around some woman who's invariably in love with him for no good reason.
So, as you might've figured out, SOS is pretty alienating. Not even the wackiness of writer Steve Gerber can save him from his aloof unlikability, nor can moving him cross country from Massachusetts to (hilariously) St. Louis, where he battles all sorts of kooky characters like the Lantern Guy from the Stairway to Heaven Poster, who talks in hippie jargon and runs a group of anachronistic nihilists, who believe in nothing.
SOS does reach his peak here, though, in a string of exorcist-themed issues that really milk the atmosphere and motifs of The Exorcist (the movie) for all its worth. There's also some pretty nifty-designed tarot-themed issues that got me into tarot again for a couple of days. I also think it's great that SOS has a hip divinity student for a sidekick for awhile.
But then Gerber leaves and John Warner helms a confusing reboot full of too much new age mush. Every issue turns into a confusing psychedelic passion play which is pretty cool to look at but a chore to read.
Now, Satana.
Satana looks like she stepped out of Barbarella, complete with leg warmers and a ram's helmet. She's a succubus, and she sends the souls of men she, uh, sucks, to hell, in the form of butterflies, which is really neat and hippyish, even though for aesthetic reasons I wish she ate them instead. She's a breath of fresh air after her brother, because whereas SOS is always in serious torment about the state of his soul, Satana is just unabashedly evil and doesn't give a fuck. By happenstance she does good sometimes, such as in one story where she befriends a group of satanists in order to use them for her own nefarious means and actually ends up saving one of their souls when the girl dies trying to protect her new friend (Satana). I love it when comic books tackle characters who by ambivalence or chance actually cause good things to happen (see Conan, The Hulk, Man-Thing).
Unfortunately, Satana is more cool sleazy concept art than a coherent character study. She has too many writers that want to do too many sundry plot arcs with her, and while the art remains consistently awesome, the stories are a jumbled, tedious mess (the text stories are especially wretched). I can't overstate how extremely annoying these stories are: any time you get a handle on who her enemies/cast of characters are, a new kid comes along and wrecks everything and we're back to scratch. Chris Claremont has some interesting ideas about how to turn Satana "straight," but in an included essay even he admits they were poorly executed. It's pretty frustrating to think of all the missteps with this character due to either aloof European artists missing deadlines or missing their air mail.
TL, DR: Wasted potential!