I want to quote a review from the Asset Store that I think fits well in this discussion:
RAD templating is king
Having come from a background of over 15 years of business application development and 25 years of hobbyist low level (assembler) game development I decided to embrace this new golden age of game development platforms and after carefully evaluating my options I selected Unity. I was then a bit shocked to discover there wasn't much in the way of structured document processing assets for Unity, and absolutely none in Unity itself. Time is an unfortunately immutable asset and for individuals/indy teams time is almost always an extremely precious resource, so being able to swiftly iterate design tests by creating UI templates on the fly should be a principal requirement for game developers and asset developers both (not to mention as a part of Unity itself!).
Having found the native GUI unusable for even moderately detailed layouts and some paid assets, while fast to render, were slow to design and had no provision for file based templated design. I was about ready to write my own document parser based on some open source C# HTML/CSS libraries, which would have been a huge time-suck, but with a view to developing within Unity for X years, should show a return along a deep enough time-line. I then stumbled across noesisGUI, which after a day of testing the features promptly ticked all the boxes for me. I had to learn XAML/WPF/Blend, but the learning curve time investment now pays for itself by allowing me to drop in template elements previously written or designed by hand with Visual Studio/KAXaml (just text, you could use VI if you felt like it, loldont) or even automatically built with web/server based tools, because it's based on standard flat file documents, not manually created elements that have no standard document file source. You can of course build an entire interface purely from code as well, but obviously that's a bit painful, however if you have previously developed your own document parsing system to work around Unity UI design limitations you might use that to convert existing work.
There is a wealth of documentation for XAML/WPF and the noesisGUI docs cover any shortfall in understanding any Unity specific processes for usage that won't be covered by generic docs. It's also quite intuitive so I found myself not having to refer to docs after a very short period of usage. There is a public and active forum and of course example usage for Unity to get you started.
Performance of the system seems very good to me, although it's pointless comparing to the native Unity GUI, except to say it's blindingly fast in comparison, but a scrollable window containing an animated "inventory" system with action bindings on hundreds of items draws in a few milliseconds with 1.1.13, and that's with no optimization in design at all, just a simple test. You can nest multiple documents (a feature you might expect, but it must be noted, as very little is at it should be in game development land) and of course apply style sheets.
I would advise any developer currently using UI based design tools to iterate their interfaces to consider the long term investment of using a document based approach and i highly recommend noesisGUI for this purpose.
And one more talking about business applications:
Coming from a history of LOB applications I've had the pleasure of trying many different GUI kits and technologies. The best one (for me) has been WPF/Silverlight with its XAML based approach, so when I saw someone was bringing this approach to Unity I had to try it - and I am impressed. NoesisGUI is now my preferred GUI toolkit for Unity, and the only thing holding it back from using it on all my projects is the current lack of support for Windows Phone and Store. Fortunately, these are both on the roadmap.
The developer has also been extremely helpful with my questions and resolving my issues.
I don't really have any interest in the next iteration of Unity's own gui anymore. This is it for me.
In my opinion NoesisGUI offers a set of default controls and panels, and the ability to create your own UserControls, that will be enough to create almost any desktop application you have in mind. If you take a look at our ControlGallery demo in Unity, the Palette sample is a mockup of what could be a desktop application like Microsoft Blend.
The possibilities are very big given the extensibility and styling capabilities of NoesisGUI