Every experience I've had in Eberron has been great with one exception - a game that was both in and out of Eberron.
It is a great setting, but it is best done with a little extra preparation. I've seen a lot of people ask about how to convert certain adventure paths into Eberron modules ... and to an extent, that misses the point of Eberron. Eberron is a setting that asks for a different approach - a noir approach. Moral ambiguity, tough choices, and lots and lots of pain. That gets mixed in with a healthy hand of pulp adventure and action to give you a great - and unique feel.
Most adventure paths are too black and white for Eberron to really shine. Just dropping PCs in Red Hand of Doom set in Eberron can be done, but it is like using a script for Hercules the Legendary Journeys on Game of Thrones.
The Great News for Eberron fans is that the recent pdf release, combined with the older 3E materials, gives you pretty much everything you need to run a game except some monster conversions - and those are pretty easy to find online or do yourself.
The one game in Eberron that was not so great was one where a DM took a stock module and ran it in Eberron. It was a fun adventure, it was run well as a D&D game... but it did not bring in the elements of Eberron that make the setting unique. That left all the fun story hooks that PCs had in their backstory dry, and left all the mysteries of Eberron unexplored. It just felt wasted and it made a few of us want to leave the storyline behind and explore a world he was not ready to have us explore. That was frustrating, despite it being a well run stock adventure.
Every Eberron campaign world is unique as DMs take the big mysteries of the setting and implement their own resolution to the mysteries and their own way to develop the storylines. It is a real experience when done well... but it just isn't something that works best as a casually run module with little prep. It shnes when the DM done the research, found the hooks that speak to them, and has planned a future for the adventure.