I like the idea of them not all being perfect - Camille's spells don't always work, Delilah turns into a house cat instead of one of the larger cats. Not sure Menolly, since the book was so focused on her being a vampire. I guess I'll have to wait for her (first) book, Darkling, to find out.
It's also nice to have fae (half human though they are) as the foucs of a series. The only other one I can think of is Laurell K. Hamilton's Merry Gentry series (book 1 is A Kiss of Shadows]), and Merry is also half human. The idea that them being half humans has a negitve effect on their magic is a new one, but now I find it odd that it is a new one to be given how much sense it makes!