I may be wrong but im pretty sure that everyone within the area of grease is treated as flat-footed unless they have 5+ ranks in tumble.
I did miss that bit about being considered flat-footed while balancing. I suspect lots of people miss that since it's not in the spell directly. That said, at higher levels can't larger creatures just step out of the area or magical creatures just fly off? (playing the gadfly here
What do I win?
An awesome-five for mentioning Penny Arcade's On the Rain-Slick Precipice of Darkness.
Flying pretty much negates Grease (though if the flyer is standing on the ground when you cast it, they're still affected until their turn), as does having ranks in balance, and uncanny dodge to some extent.
It's also one of the better spells for Chain Spell. Chain that grease, and put it on the BBEG cleric's holy symbol, and his right boot, and his left boot, and his weapon, and the feet of his minions, and, and, and...
There's also the fact that it shuts down a large portion of charging type enemies, making the terrain very difficult to charge through.You can cast it on yourself/party member if it's a creature with improved grab/grapple.You can cast it on an item to disarm a foe or to stop someone stealing the quest macguffin.Hell... you can put it on squeaky hinges and wagon cartwheels in your downtime.... or coat your enemy's wagon brakes so they don't work, or his wheels so he crashes when he gets to a corner.It doesn't have a duration so you can fill a 20 foot pit traps with 10 foot of grease... good luck making that climb check, or having a friend pull you out.So much utility in combat and out... and "scales" with level better than some piddly magic missile or lesser orb damage. It's well worth a spell known / prepared slot.
Actually, it has a duration 1 round/level.