Previously in Diablo 3, many players would like to farm in Whimsyshire. As time goes by, Whimsyshire is not that popular any more. Recently, a player asked on the community if there were still players farming Whimsyshire. Community manager Lylirra said Whimsyshire was a place of fun. She will go there. But it’s not a farm spot.
Whimsyshire is actually one of my favorite places to farm whenever I'm looking to gear up another character, or help out a friend do something similar. It's nice change of pace (and scenery) and, hey, I get to punch bears in the face. So there's that.
As for super serious farming? Nah, not really. It's definitely not my preferred spot, but it wasn't really designed to be that. It's a fun addition to the game, for sure, but not necessarily a core part of it. :)
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