My own experience is quite the opposite - Clang's diagnostics are both more useful and precise, and there are also more warnings about potential mistakes. Perhaps you used an old version?
I maintain a medium-sized proprietary application, a few million lines of C and C++ code. We use GCC and are mainly happy with it, but do build with Clang from time to time, just to see if it catches something that GCC didn't - and it often does. I also sometimes use Clang for particularly messy C++ work because of its clearer diagnostics.
Interestingly, Clang builds our code base slower than GCC. This is most likely because our compile times are dominated by a few very large (>100000 lines) machine-generated C files with very large functions, and apparently Clang doesn't handle this quite as well as GCC. For more reasonably-sized files, Clang is faster.
The quality of the generated code for our purposes (branchy integer code, very little FP, nothing vectorisable) is comparable between the compilers - the difference is usually not significant.