Good quality, stable and rocking. I use it on daily basis.
Would get my 5 if error checking and mistake-proofing were enhanced. Also support on windows is not that good.
Being a cross platform compiler, GCC is often the compiler of choice for developing software that is required to execute on a wide variety of hardware and/or operating systems.
As most of the compiler with GCC, is the same on every platform, only code which explicitly uses platform-specific features has to be rewritten for each system.
It has also been ported to more kinds of processors and operating systems than any other compiler.
There are quite a few Ada compilers floating around on the internet. The only Ada compiler that's actually free (libre) is GNAT. It excels at everything from error reporting to generating excellent code.
With it's inclusion into GCC, it's getting even better. So sure, the GPL exception's gone but who needs it anyways?