Several packages on the Unity asset store have tried to build a more fluent UI creation system. They have met with varying success and all suffer from one underlying issue, they weren't built by Unity and don't have access to some of the underlying runtime and rendering components of the Unity editor and player. This results in some performance issues (but in some cases they are actually faster than the legacy GUI, especially on mobile platforms) and (in some cases) hacky workarounds. All in all, most have done incredibly well without access to Unity's innards.
With the release of the new Unity UI system however, I would recommend checking the state of many of the GUI assets out there as several (quite understandably) have bowed their heads and are dropping off the store. The main package is still ongoing is NGUI (http://bit.ly/UnityAssetStore-NGUI), but it does have a hefty price tag to it. With it though are a multitude of supporting assets to make its adoption easier and offer integration into several other assets.