I've used it. I managed to get by OK, although I don't love C/C++ the way I used to (and mind you, one of the earliest C++ development systems for PC systems was my product!).
In actual fact, I don't presently have the CDE installed and haven't for some time, since I do stuff in C so rarely these days it's not worth my installing and maintaining it. Easier for me to just edit in Emacs.
An IDE is no substitute for an Intelligent Developer.