as has been discussed, the whip becomes most useful when used by a dual-wielding combatant. i had a ranger who was a whip specialist (with a ligh mace in the off-hand), and it worked like this:
1) ranger trips opponent (no AoO bc of Improved Trip)
2) ranger thwacks opponent on head w/ mace on off-hand attack (almost guaranteed, bc opponent is prone)
3) ranger thwacks opponent on head w/ mace again bc: "If you trip an opponent in melee combat, you immediately get a melee attack against that opponent as if you hadn’t used your attack for the trip attempt."
4) when opponent rises from prone position: AoO... thwack!
that's three thwacks for the price of one!
EDIT
hmmm... re-reading the rules, i realize that to gain the benefits of dual-wielding, you must use a full-round action... can trip be used as one attack in a full-round action? if not, i must've taken some feat that allowed it somehow. i was a neonate back then. i can't remember...