Using this probably depends on how much risk you can live with. Personally, I've always hesitated turning on extra optimization during compiles. For example, on AIX there is a caveat in the man pages for the built-in compiler of "The -O3 specific optimizations have the potential to alter the semantics of a user's program", which doesn't give me any warm feelings.
I'm not saying gcc is subject to this since I haven't used it in a while, but the fact that they don't enable these cool features by default seems to indicate they understand there is some risk involved.