I know I'm a little late to the party here, but I finally got my copy of the Tau codex last night, so I thought I'd post up some brief thoughts and impressions.
To begin, this is actually the first 6th Edition Codex I've picked up, or even really looked at. While initially I was annoyed with the idea of hard-cover codices as a simple price inflation technique, I can't deny that the book itself is nice, and feels to be of higher quality than your typical soft cover codex. That said, it is a gaming book that's going to get punished to some degree or another. Having it be hardcover should make it more resilient, but also will at least make me more guilty when I drop it and bend a corner.
As for the contents, I obviously have not yet read the book cover to cover yet, but I am pleased by the fluff section so far. As primarily a Tyranid player, I'm used to a lot of the fluff being written from an Imperial persepctive, which isn't always that satisfying. In this instance, the background felt much more complete and fleshed out than the two previous Tau codices.
As for the rules/army lists, I haven't looked into everything, but I did think the point value changes were interesting, and will play a part in how I structure my army if and when I make it. The pulse carbines got a lot better being made Assault 2, and makes firewarriors with them more viable. Also, stealth suits are absolutely nasty in cover with essentially a base 4+ cover save in the open. They also are boosted by the fact that both the Burst Cannon and Fusion Blaster are better, the former getting more shots and the latter getting a longer range.
While the points for firewarriors and some others dropped, I can see my Commander builds becoming much more expensive. Granted, this is partly because I'd want to play with all the cool toys. The suits come standard with multi-trackers, so that HQ squad can put out a lot of your favorite kinds of fire.
Pathfinder teams became even better I think, with more flexibility since they don't have to take an expensive Devilfish with each squad. At a base of 4 models, with 3 able to upgrade to Rail or Ion rifles, you can get some nice cheap sniper units, though they are at risk of giving up first blood. Meanwhile, your bigger squads are still nice with markerlights to ensure a high-priority target will go down.
In the end, the new Tau got a lot of cool toys and options, and became a lot scarier to hit if they're castled up. In the coming weeks and months we'll see how the armies shape up and adapt and find what builds are terrifying and overall where this codex ranks in terms of power.