I started saving up well in advance so I got all the sets last week :)
So far, I've built the Destiny's Wing, Throne Room, Stormbringer and Firstbourne sets. I'm saving the Oni Titan, Dieselnaut and Dragon Pit for last.
My thoughts so far:
THRONE ROOM
Nice, but mainly for the minifigs, particularly Skylor and Harumi, both of which were exclusives until Harumi also showed up in the Oni Titan set, which was revealed later than the rest. The throne room looks stylish but not much going on, and particularly having a prison cell which is just a front facade (with "bars" that don't even reach the floor) doesn't make much sense.
DESTINY'S WING
Very nice little build - not your default aircraft, with the lack of a proper nose and the wings at the very front of the craft instead of at the middle/back. I like that whole "built from wreckage" look of the flyer, Kai's tattered look is nice and Jet Jack is a cool minifig.
STORMBRINGER
I was actually pleasantly surprised by this build, since the pics really don't do it justice. The head looks a bit blocky but the build of body actually arches up and down a bit, giving the creature a very serpent-like look - it reminds me very slightly of the winged Fell Beasts from the Lord of the Rings movies. I also like how they built the spring-loaded shooters into the creature's mouth, and it looks appropriately scaly.
FIRSTBOURNE
Another awesome build and I was afraid that, building it right after having built the Stormbringer, it would feel repetitive but don't worry - it really is its own thing. Again great detail with the horns, scales and sweeping tail, and the creature looks much more streamlined than previous dragons. Some very interesting techniques and parts used to great effect. A bit of a weird choice to make the inside of the mouth grey, instead of red, and I will never be a fan of the use of plastic on a frame to form wings, but here it just works better than any alternative.
Generally, as someone who has never been a fan of the Ninjago dragons (apart from the green mech from the Movie), the dragons themselves were the most positive surprise. They look much more like animals and less like mechs, all thanks to a few simple touches - more streamlining, more details (despite hardly any stickers) and especially the use of printed pieces for the eyes, giving them a much more realistic look than by using an unprinted brick.
The sidebuilds are nice enough and particularly the Huntercopter in the "Firstbourne" set is a nice-to-have since it doesn't look like any other helicopter thanks to its angled rotor. The minifigs are nice - the Ninja are what you expect, just more tattered, and the Dragon Keepers are very distinctive and detailed, even though it would have been nicer to have a few more different ones... Even after having built just 3 sets, I already have two Jet Jacks and two Muzzles.