Just catching up with the game now it's released.  Some comments from some media outlets:
Joystiq (2.5 stars/5)
"I'm pretty sure that if I could get a peek at Castlevania: Harmony of Despair director Koji Igarashi's Netflix queue, I'd find that his most recent rental would be the 1993 comedy Groundhog Day. Virtually nothing is explained,  it reuses background tiles and sprites from years-old games and is, frankly, not fun at all if you intend to play it alone. Save your $15.
Played with friends, however, Harmony of Despair is actually ... kinda fun."
Eurogamer.net (4/10)
"I considered what a joy it would be to play a Symphony of the Night-type adventure with a few friends, exploring a castle that had untold hours of secrets to unlock, each of us developing our own fighting styles from an array of possible strategies ... that would be a fantastic journey. It's what this mess of half-ideas and compromises wanted to be."
Kotaku 
"There's a brilliant idea behind Castlevania: Harmony of Despair, but its execution is disappointingly lacking."
IGN UK (7.5)
"I applaud Konami for trying something new ... but I'd have to say this Castlevania experiment is only a modest success and there is definitely room for improvement. The focus on multiplayer came at the cost of the single-player experience. With so little of the game explained, some players may find themselves confused or, worse, frustrated. ... because assets seem to have been plucked straight from previous games, that $15 price tag is questionable."
Average reviews, but they're all saying it'll appeal to fans of the series.  I agree with the principle of a multiplayer Castlevania, but it just all seems a bit lacklustre.  I don't fancy playing flat PoR type level-design over-and-over again, even if there is a bit of excitement in teaming up and downing bosses.