I think OC's can work well as long as the setting isn't smack dab in the middle of the fandom's main story and they insert themselves in it to be made really important somehow. The OC's that just want to be romanced though? Yeah, lame.

I already told you before Naruto spawns the worst characters imaginable, makes me sick just thinking of all the overpowered boring characters I've had to read through. Is there really such thing as a bad Harry Potter OC though?