As somebody who is learning C++ and decided to go with clang, the error reporting is absolutely phenomenal. It tells me not only what line errors are on but also makes suggestions as to how to fix it. I don't ever recall GCC having very helpful error messages.
GCC has nested functions, which most GCC users hate. Clang has blocks, which are at least widely used on OS X at least and proposed for standardization. I'm not sure this is a point in GCC's favor.
As an Objective-C (iOS) developer, I really love blocks. I thought the syntax was kind of weird when starting to learn it, but it's grown on me. I really hope ObjC blocks will become part of the C standard someday. If I remember people have proposed C++ lambdas to become part of the C standard as well. It will be interesting to see which one (if any) will win in a standardising process.
Is that C language features or compiler-specific extensions? Arguably, sticking to the standard is actually the better alternative here in terms of portability.
