DISCLAIMER: I have never played the original game, but I have watched a LP of it.
Story: The story is typical fare--a bunch of highschoolers get shipped off to a creepy place dripping in atmosphere. Now what, pray tell, does this anime in particular do to make this interesting?
Well, they do make it pretty scary...
Truth be told, this is one of those anime that will, at times, manage to make you pause the episode, turn on all the lights, and keep watching. However, "at times" refers to about two or three scenes in the entire anime. The anime does really well when it ups its already well-done atmosphere in such a way that will make anyone's heartbeat speed up. But the anime doesn't do this too often, especially since it later trades the eerie moments for gore-fests.
And what gore-fests they are.
Holy crap, this anime is disgusting. Blood (I can handle this), intestines-ripping (I can handle this), decapitation (I can handle this), tongue-cutting (I can handle this), eye-stabbing (HELL NO), anything goes. I watch and read quite a few violent and bloody things, but this anime is just crazy. Usually, this type of anime is heavily censored, but I easily found an uncensored version of this anime. (This is not a bad thing at all, because there's no stupid shadowing or pixelation.)
And, while the gore is definitely a defining point of the anime, it is a little too obvious and kills the dark atmosphere of the show and almost makes it ridiculous.
Oh, and speaking of ridiculous... this show is a (very mild) ecchi. Yep.
Finally, the "bad end" feel of the show (without spoiling, people die) could be a turn-off to some fans of the game.
Animation: The show itself looks quite nice. The color palette is near-excellent, with everything draped in darkness. The anime also takes advantage of interesting camera angles and techniques. And, of course, the quality of the gore is to die for (sorry, sorry).
And now, for the bad.
Out-of-place fanservice. The anime chooses to show pantsu at the most terrible moments--the last thing I want during a creepy, hide-your-eyes-behind-your-hands scene is a panty shot. Seriously, this show needs to learn the meaning of SUBTLETY IS NOT A BAD IDEA.
In addition, the character designs are bland and the differences in hair are the only way to tell people apart. This is somewhat an issue created by the original game, but the anime does nothing to improve on it.
Sound: First and foremost, the OP is beautiful. I love it. The ED is "meh" in comparison. There's only one itty-bitty problem--this is one of those OVAs that would benefit from being a longer series. Scare after scare, murder after murder is not the way to go if a series wants impact. So why is there a (rather lengthy) OP and ED for each episode? Whatever.
The OST is fine, and adds nice touches to the scenes.
But the voice acting is actually quite good, even though some of the voices are so annoying you'll want to punch your computer screen (YUKA) or you won't care when the characters scream (YUKAYUKAYUKA). Each seiyuu manages to fit his/her character well, which is great and all, but I wish there would be more silence in the anime. It feels like every second is filled up with some voice work, whether it be a scream or a gasp or anything.
The voices during the torture moments? They are ready to disturb you for life.
Characters: Four episode OVAs cannot achieve much character development. They just can't. Four episode OVAs which kill off characters like there's no tomorrow cannot achieve any character development.
In addition, nearly every character is an IDIOT. I mean, I get that they're all in a bad situation, but DAMN.
Mochida Satoshi: He is the typical shonen lead. That's about it. There's a reason he's not exactly a super-popular character (outside of the series. Within the series, it seems like every girl has the hots for him).
Nakashima Naomi: I have a friend who likes Naomi a lot, and it seems that she's rather popular. So let me be blunt--I hate her. She's annoying and whiny, and I get that "OMG bad stuff happened to her OMG OMG how can you be so heartless she's so strong OMG" and guess what? I don't care. She's just another tsundere who can't handle herself and needs to man up. Geez, woman.
Kishinuma Yoshiki: I don't hate him. Not only is he actually a go-getter who's caring and kind, but he's also an effing BADASS when he wants to be. Oh, and he's hot (especially in the manga). Once again, though, he is a static character.
Shinozaki Ayumi: Hats off to her for getting stuff done. Hats back on because she likes Satoshi. I'm not criticizing her taste in guys (even though the guy with her is way hotter and cares for her a lot), but rather how it defines her actions. If this series wanted to make Satoshi the main guy in a harem, they should have done that instead of making a horror.
Mochida Yuka: Satoshi's good-for-nothing loli sister.
Shinohara Seiko: Another perverted comic relief. Hey, studios, please STOP MAKING THESE CHARACTERS!
Morishige Sakutarou: I hate this guy, but I hate him even more because the anime leaves out an important point about him from the game.
Suzumoto Mayu: I hate her, too. But she does wear shimapan, if you care.
Shishido Yui: Caring and protective teacher type. Wait, am I not supposed to be spouting tropes?
Naho: Very likely the only female character I like a lot. Her character wasn't fully explored in the anime, but she's still likable.
There are other characters (most notably sexy beast Kizami), but I won't go in depth.
Overall: If you want gore, then go watch this right now. If you don't desperately need your blood quota for the day, try out the RPG or manga instead. This anime feels like a tired adaptation that, if executed better, could have been improved immensely.