Overall, I liked it quite a bit, but I think that it might have been better received if all three volumes had been released together or if some of the things revealed in Part 3 had been worked into the earlier volumes.

Like you, I don't think that it's too majorly off kilter, but I was rather on the edge before Part 3 came out, since the explanations for the things I did find out of place or OOC were mostly given in Part 3. ASN's latest interview with Gene Yang also explains what they were doing with Aang making that promise to Zuko.