I'm also a fan of the Bitterbynde series and happily have a signed copy of the series from Celia whom I've met a few times.
Like some of the previous comments here I think the prose has a whimsical lyricism to it framed as it is within a folkloric 'old-style' linguistic narrative. I like the meandering style that lends itself to some lovely descriptions but I agree this is an acquired taste and not for everyone.. It also helps if you enjoy Scottish and Irish myths populated by Faerie folk. You can certainly see the influences (as Cecelia has previously stated) of Shakespeare, George MacDonald, Tolkien, C.S. Lewis, Andrew Lang, Ursula LeGuin, Andre Norton, Susannah Clark, Charles Kingsley Allan Garner, William Morris and Dianna Wyne-Jones.
The plot is interesting but could have been better edited in parts and I didn't think the final installment was as well constructed as the first and second books. It's over 10 years since I last read these but I seem to recall there was some criticism towards the passive nature of the female characters.
I think I've mentioned this elsewhere in the past but it's interesting to note that her initial success came as a blog writer, who based on the strength of same extracts from the Ill-Made mute she had posted, resulted in a U.S. editor who had read her posts precipitating a late night phone call from a Time Warner executive who literally offered her a $200,00 plus contract on the spot for development of the books. To her good fortune she had already largely written the 3 books in this series..so you see there's hope for us all...
I should say that I did not like her follow-up Iron Tree quartet as much. I feel the Bitterbynde series is the best thing she's written to date.