As an avid Avatar and Legend of Korra fan, here is my input.
Is it perfect? No, of course not.
Is there room for improvement? Absolutely.
However, it's certainly clear that they put in a lot of effort into this, and tried to keep it close to the original story with giving it more of a realistic, dark theme. Again, fans will certainly notice the changes, and the combining of characters / episodes which made me go, "what? Why have they done this?" But on further thought, it made sense since they only were permitted 8 episodes, corners had to be cut.
But I think they did a very good job with it. Some new small details were added, which made me emotional (like Zuko and his crew, I teared up, giving Zuko's character a little more depth), I teared up with Iroh also.
Overall, I think fans will very much enjoy this as I have. I hope for a Season 2!