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Is filler a good or a bad thing? On the bright side filler adds monsters-of-the-day that usually are a treat while we get to see cute characters-of-the-day that would otherwise NEVER be seen. Filler can build character personality and make a certain character or characters seem more defined and appealing. It can also be used for comic relief or other plot devices.

On the down side the characters they introduce never usually get much more recognition, aren't usually well explained, and sometimes can get repetetive and feel like they're slowing down the story. Sometimes it just dillutes character personality, showing little defining characteristics. Other times it's so boring that you may just skip over the entire episode, since it has nothing to donate to plot or any valuable subject.

What's your stance?

Tough to say, filler can be good, and filler can be soul crushing, time wasting, can-you-please-continue-the-story-already-and-stop-stretching-out-this-manga/anime-so-you-can-make-more-money-with-more-episodes/chapters.
I can see if a manga or anime needs many episodes to tell it's sprawling story, but sometimes it's just useless filler that drives the viewer nuts... I would say a good anime or manga can provide a decent amount of filler that not only is entertaining, but might actually make the story move along, even if it's only the heroine and hero coming closer together.
If anything I'd say a good amount would be one or two filler episodes, followed by a regular episode, and then or or two filler episodes again and so on.
But filler is the reason I can't watch Naruto or Bleach, or read any of awesome Rumiko Takahashi's awesome mangas... they just go on forever it seems... 30+ volumes of any manga just seems crazy. And the hours it would take to to watch 200+ episodes, and still be interested...