All long-running Animu's that are based on Mangos run into too much filler; Mangos take too long in comparison so invariably an Animu will catch up.
What makes a good Animu from a bad Animu is how the producers handle the filler; One Piece handles the filler supurbly, by taking throwaway side-stories only mentioned in the Mango and exploding them out to full mini-arcs, they've done this so well in fact that i had to have several of the filler arcs pointed out to me.
Bleach started off badly with the Bounto arc, which they tried to pretend was legit and canon and generally sucked big hairy gorilla bawls. They then seemed to leap onto a good thing with the Amagai arc, which came out of nowhere and was literally the producers going "Hay gies, sorry, it's time for some filler again. It's not canon, it's just filler, but hey, that's life." and it started off real good too, until Amagai ended up horrifically stereotyped into a bad guy and the arc was ruined.
The latest filler arc (The Soul Slayer\Zanpaktou (did i spell that right?)) looks pretty good, the art style has had a marked increase, it was starting to slide a bit at the end there but with the latest arc it shot right back up, for awhile, it seems to be slipping again which is a real pity, but so far it's an interesting story and again they've taken a "Hay gies, filler time!" approach, which works just fine.
We'll see if they manage to fuck it up like the Amagai arc though.