For me, speed isn't the only thing I worry about. Intel's debugger is way better. I find the Intel compilers diagnostics, remarks, warnings, and errors to be helpful to me while building my app. So, I say if you're on Linux, pick up you free copy, and enjoy building with the great debugger and compiler output. Also I find the Intel compiler much faster at building unoptimized debug builds, which is another big plus. Then when you're ready to release you can play with both of them to see which one likes your code more.
Of course if you need any money for your app, then you can't release with the free Intel. But because it's so good for developing, I'm always gonna build on Linux with the free version. Every once in a while I'll run GCC over my code to make sure it's still happy.