Wikipedia has this to say about the leanan sidhe (yay for quoting the Intarwebz!):
"The leanan sídhe is generally depicted as a beautiful muse, who offers inspiration to an artist in exchange for their love and devotion; however, this frequently results in madness for the artist, as well as premature death."
A thought I've been kicking around for a while now though, is that the depiction above is a rather unfair one. For one thing, it isn't just inspiration she offers, or at least, it doesn't seem to be. It's both skill and inspiration... and then they die young, her chosen artists. But how good you get as an artist also depends on how much time and care you put into your art - same as any other skillset. In other words, artistic skill (if not necessarily inspiration) is what the artist gets in return for parcelling away chunks of their lives to their muse.
Outliers by Malcolm Gladwell mentions something about how it takes most humans 10,000 hours or so to attain a highish level of mastery. That's 10,000 hours of your life you'll never see again.
So what's the leanan sidhe really doing? Stealing the lives of her 'chosen', and dooming them to early splatting? Or is she merely compressing the whole process? Maybe that's the real bargain she offers. ;)
'Listen, puny mortal, you can spend the rest of your life improving your craft, or you can just pay me upfront, now, and I'll give you all the skills that would have accumulated in the time you've just bargained away. And I'll throw in some inspiration as a plus!'
...suddenly she doesn't look all that evil anymore.
>.> Especially since her way is the easy way out. You don't get to procrastinate your life away instead.