Pretty much every manga I've bought on a total whim has turned out badly, ES being the exception. Some of the worst:
"Hanami: an international love story". Dude, the word international got me. I'm not a shojo person generally, but the plot wasn't that original and the love story revolved around two people who couldn't even communicate effectively. Granted, the premise could have been interesting in the right hands, but... it wasn't.
"Claymore": Meh and meh. Look, I'm an emo monster, guys. And a hot chick. And I have a huge sword.
"Grand Theft Galaxy" This had potential. It really did. That is, until the prissy, uninteresting main character showed up. Yeah, it's supposed to be fish-out-of-water, but that's only ok if your mc has some redeeming characteristics. Anything, really. The minor characters are pretty epic, though.
"Penguin Gin" Ugh. I liked penguins, ok? I bought four volumes of this. FOUR. I'm a little ashamed. The girl-love-interest and the guy go on like half a date, then he dies and gets reincarnated as a penguin while he tries to regain his body or get reborn or something. Without knowing he's alive, the chick inexplicably stays devoted to him. She's known him personally for like 5 hours prior.
Oh, then the PENGUIN becomes her pet. In Japan. I'm pretty sure that's illegal...
"Bleach": I have until 20-something, then I quit. Kubo ruined or phased out the characters and plot elements I liked. 'Nuff said.