As a system, Warbirds is fast, light, and gets right to the point. You're a pilot, you have a plane, go have some adventures.
I am particularly fond of the way characters are handled inside and outside of the cockpit. On foot, a pilot may fulfill just about any character concept the player has in mind. It's really down to the GM whether "botany" constitutes a viable skill in the context of the campaign, or how best to statistically represent a martial artist character. However, when these characters step into the cockpit, it is the sum total of their statistics that determines their piloting ability. In this way, a player group can be extremely diverse but still possess similar "power" levels in the air.
I also quite like the way the Trait system customizes players' planes to perform better in either dogfighting or strafing attacks. It's possible to create extremely durable fighters, extremely frail (yet supremely destructive) ground-attack planes, or anything in between. I've been playing around with a concept for a plane that specializes in head-to-head aerial attacks by loading it with machine guns and perks that reduce head-on damage, then attempting dangerous stall-out maneuvers using the "stunt" system. It's not likely to be a particularly effective design/playstyle, but I appreciate the flexibility.
I wouldn't say this is a system particularly well-suited for grim or gritty campaigns, but it's great for four-color action stories centered around flying. You spend a little bit of time talking, maybe shoot some mooks or go out drinking/carousing, and then you're back in the air for some high-stakes dogfighting. The system covers many of the most important aspects of being a fantasy pilot: the fame, the customization, the rivalries, the nicknames/camaraderie, and a serviceable setting that emphasizes air combat.
As a book, Warbirds have a couple small shortcomings. I noticed a few spelling/grammatical errors as I read through the book. It's neither a deal-breaker nor anything particularly egregious, but it was a little disappointing. I also might have laid out the information a bit differently, as certain sections tend to break flow. Warbird customization in particular was a sticking point, as all of the "traits" are pushed off until the end of the section. It was a bit jarring because the book teaches you how to build most of a Warbird, tells you to pick some traits, shows you a list of traits (but not what they do), and then moves right ahead into the next step. It is only when you skip ahead or reach the end of the section that you can see what all the available traits are and how they affect your plane.
The art was something of a double-edged sword. I really liked what was included, but I was left wanting a bit more. There are some short well-done comics, many slice-of-life scenes, and some character portraits, but precious few detailed fighter designs. The statistical guidelines for each nations' fighters lacked any visual reference, and the only named design I can recall (a Santiago fighter) lacked any accompanying statistics/traits. I would really appreciate some kind of 'Hangars of Azure' supplement that provided detailed examples of various fighters, bombers and Warbirds; so that I can provide my players with more substantial descriptions/visual references. Without traditional area maps, miniatures or even reference artwork, I worry about how I'm going to represent this game visually to my group. It is worth mentioning that the Warbird sheet includes a "Nose Art" section, at least covering one of the more personal aspects of aircraft livery.
Overall, I'm very satisfied with my purchase and I'm excited to run a campaign with this system.