Since switching to a build without the 51 point Beast Mastery talent I decided I should try the slowing capabilities of the Hyena, since I loved my Chimaera so much. To get straight to the point, the Hyena slow does not compare to the Chimaera slow at all.
With Longevity it is on a 14 second cooldown (6 second slow). So theoretically it should be up 43% of the time. However, it is a physical ability and therefore can be dodged, blocked or parried. This means against Rogues, Warriors and Feral Druids (melee classes), this slow will not be up reliably. Against casters this slow will fare better.
Compared to other pet specials this is not a top ten ability. The only real good thing going for the Hyena is that it is a Ferocity pet, which allows it to crit (with talents) reliably in PvP. However, there are other Ferocity pets out there that are better than the Hyena.