Just finished the back 4 episodes, watched the first 4 earlier.
Enjoyed the HELL out of it! It's not perfect, but a heck of a lot of fun.
The main and supporting characters are very well cast, and there are lots of fun cameos. Ian Ousley channels Sokka almost perfectly. A lot of the other characters seem to warm up and build into their roles, but by the end of the show, I was fully believing Gordon Cormier as Aaang, Kiawentiio as Kitara and Dallas Liu as Zuko. Many of the supporting cast had a similar issue, not quite warming up to their roles, but with less screen time, didn't fully reach it. I've seen a lot of criticism online of "bad acting", but I think it's weak direction that is the character problem. But a minor one, for me at least.
The visuals are amazing! At first, I was surprised they stuck so closely to the original designs, but they did a great job bringing the animated world of Avatar to life in live action. Some of the costuming and hair didn't feel "lived in", but was gorgeous nonetheless.
The story line condenses and rearranges the original, but there were only a few places where I felt they cut something out important . . . I'm positive there are filmed scenes on the editing room floor that should have made it in, there are a few narrative gaps or jumps. The story flowed well, if at a much faster pace than the animated show. But Sokka's romances felt too close together, but each individually worked out well with lots of chemistry. Loved the kaiju inspired climax in the final episode!
But . . . Momo got shafted! Very little screen time, you almost forgot he was there until suddenly he was! Really well realized in CGI, he looked REAL, as did Appa and the many weird animals of the Avatar world.
EDIT: Oh, and . . . much darker in live action than in animation. It was both a deliberate narrative choice but also the themes present in the original just hit harder in live action, I think.
