I've used Rescue like 3 times now. Only one was successful though. ;D
First was in Suzaku normal, first try. My boyfriend was the MT and said he can't get out of the cleave. I wanted to rescue him but on his screen he was already behind the hole while on mine he wasn't. Of course he fell through it.
Second was in Rabanastre, when I rescued a BLM who was at the wrong marker at the second boss to our marker. That worked.
Third was on accident. I somehow fat fingered and pulled my co-healer to us just like that in the middle of O12N. In that run I fat fingered a lot though and it was so embarrassing. At least the rescue was during the Omega-F knockback so it wasn't so detrimental. The healer was close enough to the middle to not get pushed down anyway, but well.

There were situations when I thought "now I could use Rescue" but before I use it it's too late anyway with the delay and all that.