Succy is good for lower levels, in fact even at times as a 70.
Once you're geared enough (and versus better geared people) the succubus becomes less and less a viable option.
The Best use for a succubus while grinding though is simply as a traveling partner on PVP servers. Even the Felhunter's Paranoia and Spell Lock nor the Stunning and DPS of the Felguard can compaire with the 1v1 options the Succubus gives - Even if it's to just get away and hide.
It's not gonna save a level 61 Riding through Nagrand from a 70 Rogue, but I still say 26-70 Succubus is your best bet when traveling in Contested/Enemy areas.