It's rarely bad and usually improves the anime. The problem is that it can only improve it so much and most of the time that's not enough to save the piece of garbage it is used on.
Just throwing out some animes that are a great example of really good animes that are improved by fanservice. Kill La Kill(or any Trigger/Gainax anime really), Freezing, Junketsu no Maria, Agent Aika, Monogatari series(or any Shinbo anime), Boku wa Tomodachi ga Sukunai , GJ-Bu, Nanatsu no Taizai, No Game no Life, D-Frag, Mahouka, Witch Craft Works, Shokugeki no Souma, Grisaia, Hyakka Ryouran and a couple more.
I like fan service, it makes bad animes watchable, comedy animes funnier and dramas more mature.
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