There've been some pretty interesting comments in this thread--just thought I'd chime in with my thoughts.
I've just recently gone on a spree, and bought all of the Eberron books since the ECS--I like this setting, and (except for EXPLORER'S HANDBOOK, which I'm not sure about yet), it's some nice, solid stuff.
Part of that is that it doesn't have the history that FR or Greyhawk has. Part of it is that it was designed as a campaign setting from the get-go, and didn't evolve out of a lot of stuff that was already around. That means, for example, that there are a reasonably small number of nations to cover... FIVE NATIONS hits the majority of the civilized world, and only has to cover five countries. Because of that, there was plenty of room to give backstory, some detailed NPCs (both singular people and general types), a handful of organizations, maybe a prestige class. And that's just about the perfect density to give a really really good feel for the places. Compare that to either FR or Greyhawk, and you see that either of those settings has trouble partially because there are so damned many places to talk about.
That said, I really wish that they would release more detailed Greyhawk material than just the Gazeteer... a friend of mine has pretty much all of the old GH stuff, but not everybody is that lucky. And even with the old stuff, you have to put some time in to adapt things. And on top of that, we have difficulty thinking about how to fit higher-level campaigning (15+) into the setting.
ANYway. The Eberron stuff is really well done, and I'm looking forward to more. Depending on how our current campaign goes, I may try to run an Eberron campaign (rather than the nearly-one-shot I did right after the ECS came out.)
(Note about the Mindset spells: I hope we see more of those in MAGIC OF EBERRON, and perhaps some back-patches to make some existing spells Mindset spells. I totally failed to notice these in my first pass through RACES OF EBERRON, but they're a great idea... Not only do they give a little more power to the memorizing classes, who get shorted a bit at times, but they also constrain those classes a bit more by making you have to consider memorizing a spell instead of leaving a spot free to fill later in the day when you have a specific task in mind.)
For the other books: I disagree with what seems to be the majority here about some of the books. The "complete" books don't really appeal to me, primarily because they have too many prestige classes. Prestige classes can be fun, but I much prefer the very small number (with detailed background material) in FIVE NATIONS to the great hordes of them in each of the Complete books.
I may be missing out a bit there, though. I'm much less irritated by large selections of feats than I am by large selections of prestige classes--primarily because a feat selection doesn't dominate a character the way a PrC level does, and because individual feats don't carry quite as much weird flavor with them. Because of that, I should probably take a second look at the Complete books to see what I'm missing out on when I discard them for the sake of PrCs I'll never use (or late any player of mine use.)
The environment books are very interesting, though I haven't bought any of them yet. (I've been on a budget the last several months, and the Eberron books were first on my list.) Even though the core rules cover a lot of situations pretty well, having a lot of detailed ideas and rules for specific environments can be very very handy. I don't know how much I'll like them--I'm recalling from glancing through FROSTBURN that there were some weird special materials and such in there, but also that there were some good bits about things like travel by sleigh. At the very least, though, these books provide distinctive background material that you can use to flavor parts of the world (any world), rather than even more character options.
MAGIC OF INCARNUM and WEAPONS OF LEGACY, I've been inclined to avoid. They're potentially interesting, but also kind of big changes to the world (Like the GHOSTWALK stuff, although I must admit I bought that book.)
In the end, I think that WotC is having a rough patch--not because they're starting to suck, but because they're having to think much harder about things they can do *besides* throw together huge masses of prestige classes. A lot of companies have done that, and WotC has done it, too. So, they're moving into the terrain of introducing more detailed images of races, of different climates, and they're introducing some big variant rules stuff (like Incarnum and legacy items.) Some of these things will be brilliant, and some of them won't be so good.
I'm going to keep buying WotC over third-party stuff, though... mainly because I've been burned with too much third-party material that was poorly designed, edited, and balanced than even the worst of the WotC material. (And also was inclined to change *too much*, which I can only attribute to a strange need to one-up WotC.)
And hell, I can always hope that some day WotC will release "The Frigging Huge Book of All Things Greyhawk". Delusions are good, right?