Fanservice is at best, something that will quickly get your attention, maybe make you laugh a bit, and then continue on without distracting from what is really important. As said before, context is everything.

At it's worse, fanservice makes me grind my teeth. The HBO adaptation of Game of Thrones is an example of this. You've got a deep story with well-rounded characters and complex plots. Sometimes, there's fanservice, but it goes hand in hand with the story.

For example, Daenerys spends A LOT of time in season 1 naked. I think she gets something like one sex scene per episode for the first half of that season. However, this is a part of her story arc so it doesn't take away from the plot. Her brother sells her to a warlord to build an alliance to retake the kingdom that he believes is rightfully his. Lucky for her, the warlord is more or less a good guy (that said, the culture he comes from isn't the nicest lot) and once they get past the initial cultural barriers, they come to love eachother.

On the other hand, there's one particular scene later on in season 1 that stands out as unnecessary fanservice. One character whose a bit of a manipulative bastard is monologueing about the nature of men and giving a bit of insight into his plan... All the while two girls are going at it in the background. It's as if the producers were saying "Look, this guy is talking for 5 minutes. Here's some boobs. Just look at those until we get through the boring bits." The character in question did own a few whorehouses, but it still felt really out of place. It took away from what was an important scene for that character.

Another example would be sex scenes in video games. Off the top of my head, I can't think of a single one that felt like more than a cheap attempt at fanservice. Poorly developed buildup to the protagonist and romantic interest getting it on for no reason other than the developers thought it would be hot and would amuse their players.