For those who had developed applications with D,
I used tango libraries + dfl and a bit of my own. Tango documentation is excellent. DFL not bad. Yes I did use tango then but tried phobos at first. Ready for big apps? depends on what you mean. In production use I have only used it for frontends and updaters. So far I've only used vim to code and entice to design gui because I couldn't find the right ide for me.
Other than the core library (in my case, Tango), I'm not using any external libs.
Tango documentation is decent. It's a large library, and I'd say there's documentation for about 80% of it, off the top of my head. And, of those documented classes, I'd say about 80% of them are complete and correct. Given the size of the library, the amount of documentation is impressive. But it's still not quite where it needs to be. In order to really use Tango right now, you have to read the source code (which is clean and well-written).
I don't know whether D is ready for big applications. My application is pretty small, and I'm pretty happy with it. From a language-design perspective, I prefer C# (or Java, to a lesser extent). In comparison, the D language design strikes me as somewhat clunky and awkward. Really, the only reason I chose D for this project was because A) I needed to be able to compile my code to a DLL which could be linked by an arbitrary third party; B) I needed my code to be portable between Win, Lin, and Mac; and C) I didn't want to write in C/C++.
I'm using Descent (an Eclipse IDE plugin). It provides reasonably good syntax highlighting and project navigation. Auto-complete still leaves much to be desired, and the integration with a "builder" like dsss is still lacking. But it's better than notepad :)
Code blocks seems to support D.
Note that any C library can be used with D, as D fully supports the C ABI. D has some limited support for C++ libraries, though not C++ template libraries.