One word: WOW!
The writers of this episode did their job perfectly here, covering a lot of topics in the series thus far. And the way things were played out kept with nonstop surprises. This episode had it all: action, plot developments, and one helluva plot twist in the ending.
This episode is a prime example of why WMDs are so feared, and why they cannot be taken lightly. Suzaku's determination not to use Nina's Fleija superweapon keeps with his character, but even he is unable to prevent disaster in the end. And his use of it has devastating consequences, both for himself and for Lelouch, who arguably also had a hand in Suzaku's action.
The fights between the best of the best in KMF combat are sheer art in this episode, displaying the talents of nearly all the major and minor combatants in the series. Lots of characters (mostly minor ones) die, but there are also a few major losses.
The main loss, presently, may be Nunnally. Whether or not she's dead remains to be seen, but I honestly still have hope from subtle clues scattered through the episode. But whether or not she's dead, her disappearance is going to probably drive Lelouch off the deep end.
The rest of the episode is what fans would expect from the series: stellar production values and excellent combat. Kallen's entry into the battle in her new Guren Seiten is borderline ridiculous, making her seem nearly unstoppable. But I couldn't care less; she was flat out awesome, killing Luciano with ease and nearly wasting Suzaku just as quickly.
Other topics were touched on in earnest in this episode. The issue of Toudou receiving evidence against Zero spells definite trouble for Lelouch, while Gino's attempts to win Kallen over work with character development from previous episodes. Rolo's desire to be Lelouch's only sibling is quite disturbing as usual, and Anya's seizure whilst fighting Lelouch only raises more questions about her. In other words, this episode had a lot to it.
I'm left to continue looking over the episode for clues as to Nunnally's possible fate, but I know I won't know anything until the next episode. I can only hope it will be as high caliber as this one was.