It's kind of a double-edged sword to provide constructive crit. In the end it comes down to whether or not the author takes it personally. I myself never take constructive crit personally, since I don't really base any characters or stories on me unless I would specifically write a non-fic, so I treat it as something I'll need to fix and keep in mind for the future.
But then there's those who go absolutely insane over it and call you a flamer who has no life, and even hunt down your stories to review them and call them stupid and worthless - well, you know how it goes.
Nobody really likes criticism, but if it helps them move forward, they can learn to like it.
Nevertheless, I tip my hat to anyone who reviews frequently with constructive criticism. Not everybody will appreciate it, but there's always those who will be grateful for it.