One Piece - My opinion had been swayed by a number of Naruto fanboys and fan sites so I was pretty anti-One Piece before even reading it, and judged it in a negative manner before I even read it. However seeing Shanks sacrifise his arm was enough to hook me.
Rave Master - I watched some of the eps on Cartoon Network and thought it was alright but nothing special. However One Piece taught me that a good series can sometimes take a while to take off, so I stuck with it. By around volume 10/11 I felt the series shot up in quality (when I later re-read the series, I felt 6 was a peak) and have enjoyed it ever since. I don't think Rave is as special as some of the more unique series like One Piece. It's pretty by the book. However in my opinion if you accept that it's an enjoyable series to read.