The best IDE i was able to find for C++ on linux is Qt Creator. Its only drawback is that it's quite Qt integrated, but with very few tweaks you can make regular C++ projects with no Qt dependencies.
Everything from the debugger to the code completion does work well, which is more you can say about most C++ IDE on linux.
The current Qt Creator (or I think its beta version) really has the best available C++ syntax support (which has direct impact on stuff like auto completion) I have ever seen so far.
The second best I have seen was in KDevelop (when configured correctly -- I remember it only worked correctly when you pre parsed the STL library).
I am also very excited about the clang development. It was developed in a way that it should be very easy to integrate it (or parts of it) in an IDE. Xcode4 will do that. And others probably will follow at some point.
Everything from the debugger to the code completion does work well, which is more you can say about most C++ IDE on linux.