I'm not a fan of fan service. Cartoon boobs don't do it for me and if I want to see nudity there are better options. As long as fan service isn't gratuitous, I can generally ignore it, but if I notice it more than a couple of times in an episode, it's probably going to annoy me. When I watch anime, I watch it for the characters, the story, the art, the music, and the immersion in something Japanese, so gratuitous panty shots or whatever is generally going to distract from my enjoyment of the former.
There are times when fan service isn't a big deal or even when it can enhance a story - particularly in comedies. I grew up during the golden age of teenage sex comedies in the U.S., aka the 1980s, so comedy and sex are forever intertwined in my mind. So it especially irritates me when it pops up in dramatic shows. Fan service, like any other trope, should only be used if it's integral to the storytelling. A comedy about a dope who always ends up in compromising positions with his friend's sister is one example where it can be appropriate. A non-hentai dramatic show about heroes piloting giant robots to battle an invasion of extra-dimensional demons is an example where fan service is out of place. A really heavy dramatic show where the main character is dying of a terminal disease and his friends and family struggle to come to terms with his impending death is an example of show where I want to bludgeon the creators every time I see a panty shot.
One thing I've noticed about anime is that creators really like to blur the lines between genres, to mix and match tropes and try unusual things, often ending up with the creation of new tropes. This is probably the single greatest and most entertaining creative aspect of anime. The downside is that sometimes it doesn't work well, and creators will overuse certain tropes because they're popular, rather than giving consideration to the actual merits of using a particular device in a particular context. Fan service is one of those things that I think has fallen into the "overused" bucket. It's a shame, and I blame the fans for it. Just because I like hamburger doesn't mean I want to eat it at every meal day after day.