Mostly bad. It's discracting, pointless and often breaks suspension of disbelief. What's more - and this is particularly subjective - it's not even sexy. A beautiful woman diving into battle, hair blowing in the wind and face splashed with blood as she roars her fury at the men crumpling beneath her blows: sexy. Sticking the camera on the ground to show her stupid undies while she's doing that: ew
I have nothing against sexy. Sexy is good. But fanservice is just pervy, and that's rarely good.
There are exceptions to that, but mostly they go back to the contextual appropriateness thing, and often also if there's a self-awareness about it. Like, say, a situation where an action woman gets caught by surprise when she's in the shower, and has to fight her way out to safety in a towel, and kicks arse anyway. That could be extremely sexy (especially, in my opinion, if they're mostly tastefully coy about showing anything), very fanservicy and even quite empowering... And now I kinda wanna watch this scene in something :I But... it should make sense; it should end fairly quickly (perhaps she might take pieces of clothing as it does along, so she gets more and more dressed as the action progresses), and it shouldn't ONLY be there to make the hetero teenage boys happy.