This is actually a fantastic feature. I was recently traveling in Europe, and without thinking logged into Facebook without HTTPS (using a third-party iframe app that couldn't use HTTPS). Within a few hours, I had a notification from FB letting me know that there was a new login from the same city I was in and a different OS/web browser combo.
I've experienced this too. Apparently, Google Chrome on OS X gets labeled Opera/Win XP by Facebook. I was paranoid for a little while, wondering how these hackers kept tracking me around the world with that browser/os combo.
That doesn't help much if they gained access to your Facebook through your hacked email account.
It'd be neat if it texted me when suspicious activity occurred, rather than just the in-Gmail alert.
I have those enabled as well, of course, but they are quite hard to interpret and I've had a few false positives.
I've had false positives on these alerts too, but I'm willing to put up with them. As long as there are no false negatives, I will be happy ;-)