OP is much fun to marathon, especially since it's a fight shounen, so it can take forever for anything to actually happen. Episode-long fights and things. (tho' it's much faster moving than some). Also, since all eps end "to be continued", it can be difficult to stop watching...
The downside is that you can lose HOURS this way...go to watch a little One Piece in the afternoon, wake up from the pirate trance and find the sun setting and your stomach growling like Luffy's...

(Guybrush Threepwood, was that a vid game?)