Well, I seem to remember reading about this elsewhere on the Dungeon, but the manga is pretty generic. It's not completely awful, but it's nothing to win any awards, either (personally, I think the Amazon review is a bit too harsh). Like I said, it helped pass time in the airport and while waiting for OoE to arrive and was entertaining. I keep hearing it was made just to help sell copies of CoD in Japan. I'll probably get vol. 2, but can't say it's highly recommended, as there's other things you can spend your $20 on. Tokyopop has some sample pages on their website, and I'm sure you can flip through it at Barnes and Noble. Each volume is $10, and the second one comes out in December.
The Castlevania comic (based on Castlevania Adventure) is pretty much the same. It's got a couple neat things with art in the first volume, but overall, I would have preferred to spend my money on something else. You can buy a collected TPB for something like $20 on Amazon, but I don't really think it's worth it. There's a few pages from it somewhere on the Dungeon that I sent out... Can't say I regret buying it THAT much, as it's another thing to collect.
I really can't say which I like more...the manga or the comic, but I'll probably go with the manga, though the comic has its moments, if that means anything.