At first, I thought it was okay. Gradually, I got less interested. By the end, I thought it was awful. The 2010 reboot OVAs are so clearly superior that it's instructive to compare them.
Hime is a princess from the land of monsters and all her siblings want to kill her. This is normal in that royal family. They have a rule that their children must kill each other until the only survivor becomes the next monarch. Anyway, Hime's going to acquire a motley bunch of servants, friends and hangers-on. Hiro is an ordinary, somewhat timid schoolboy who gets turned into an immortal for as long as he keeps drinking Hime's blood. Riza is a half-werewolf. Reiri is a vampire. Flandre is a tiny android with super-strength and a one-word vocabulary ("fuga").
Meanwhile, the show's plots tend to involve horror and/or monster concepts like zombie apocalypses, Dracula, a shark-assassin girl, etc. I'm on board with all that. You can see why I watched the show.
Unfortunately, the cast's boring. Many of them barely react to anything, sometimes to the point of seeming disinterested about anything in the scene except themselves. Reiri's voice actress sounds as if she's bored with the script and hoping to flirt with the director or something. She also does an offputting laugh. Flandre's barely sentient. Hime is an unpleasant person who's so cold and underreacting that it's actually a story point when someone gets her angry in ep.24. Then, in contrast, there's Hiro's sister, Sawawa. She's a lovely person. Nothing wrong with her personality, although she has the brains of a passing breeze. The problem, though, is her story function is to be a maid in Hime's house, to be vacantly unaware and to eat parfaits in a cafe. Her ability to respond to what's around her is lower than Reiri's. After a while, I started finding her presence in the anime almost insulting.
Of the other characters, Hiro is a pushover and a wimp, but at least he's likeable. Riza is a hot-headed, rough-talking thug and soon gets tiresome, especially when bickering with Reiri, but at least that's more engaging than endless non-reactions. Princess Sherwood is one of the most human cast members, but is also an obnoxious wannabe bitch queen who thinks she's in an underage harem anime and gets a crush on Hiro. Her comedy panda is a pain in the arse. (Yes, a panda.)
Then there's the general tone. Unfunny comedy is underlined by inappropriate incidental music. In ep.18, there's a moment where someone runs off a cliff without realising, stops in mid-air and then falls vertically downwards with a comedy twang sound. Death threats have no credibility, because you know that it's almost impossible for death to happen in these episodes. This yields a headbanging version of vampire lore, in which Reiri never has any victims and even if she does turn you into a vampire for current plot convenience, you can just get Hime to un-vampirise you again when you get home.
There are a few decent episodes. I liked ep.13, which ditches most of the regular cast for a Hime-and-Hiro journey to a ghost village, uninhabited except by murder victims. Ep.19 has some cool ideas, with a ghost ship and a combination of Hans Christian Andersen's Little Mermaid and a 21st century Greek siren. Unfortunately, though, the ideas are undermined by the show's usual sloppiness with lore, i.e. it simply equates mermaids and sirens.
In contrast, the 2010 OVAs are a revelation. It's fascinating to see what a difference can be made by a good director, dramatic camera angles and cool music. This version of the show is good! Formerly annoying characters suddenly become interesting. The only downside is the half-arsed fanservice (lots of blatant panty shots, no nipples in the hot spring). Mind you, the TV series was just as bad in its own way. (There, Riza was the designated sexy one.) These OVAs also have more characters, since the original manga ran from 2005-13 and so in hindsight the 2007 TV series hadn't had much material to work from.
In short, don't bother with this. The manga will be better than the TV series, but I'm pretty sure I'd dislike that too. A vampire who doesn't drink blood, doesn't have victims, can go out in the daylight and whose victims can be turned back to normal at will? If I wanted that, I'd watch Scooby Doo. Triffids will die when the sun comes out (ep.6). Sod right off. The show doesn't even have an ending. Occasionally, it'll have an amusing bit, e.g. the tasteless speculation about Riza's conception. (She's a half-werewolf. When her parents did the deed, did mum find dad sexier under a full moon? Would that count as bestiality?) I enjoyed the first few episodes, but once it settled into a formula I found myself liking it less and less.