This series is very much like a three course meal. The first two books were the starter that got you into it, ok but leaving you wanting more. The Iron Queen is the main course, you've finished and you're satisfied, but there is something missing. The Iron Knight is the delicious pudding, that you know you really shouldn't eat (in the same way that Julie Kagawa never really wanted to write it) but you do. But by heck do you enjoy it. At the end you're a little satiated but you almost always want a little bit more. 
 Brilliant characters, I even lightened up to Grim towards the end. This last book was wonderful story telling - it reminded me of Harry Potter the Philospher's Stone meeting the Angel episode when Angel tries to save Darla and the Buffy episode when Spike goes through his trials and gets back his soul. Very clever, very philosophical and just caught the essence of human, and fey ;-), life's so perfectly. 
 I read the whole series in 5 days, with The Indigo Spell inbetween. If that doesn't tell you how much I loved this series you'll never know.