I have always felt the primary use for Grease is of course as battlefield control...but not necessarily how that first comes to mind. Grease is great control not in the sense that the caster use it to turn things into a Slip 'n Slide for both the enemies and his own party as well; rather, that you can viably cut off areas of enemy movement, areas of enemy cover and the like.
Use it behind the enemy to make them think twice about retreating, to keep ranged attackers from falling back out of your reach, to make reinforcements think twice before jumping in. Toss one into areas where enemies could seek shelter and force them to stay out and play. Casting Grease where your enemies ARE NOT is often better then trying to knock them all down by throwing it at their feet.
Also useful is the fact that no SR makes Grease situationally useful against opponents even in mid/late game. Think huge golem with tons of DR and magic immunity...now think same golem lying on his back.
Also, never discount the last sentence of the spell description...
"A creature wearing greased armor or clothing gains a +10 circumstance bonus on Escape Artist checks and combat maneuver checks made to escape a grapple, and to their CMD to avoid being grappled."
Grease yourself as a buff of sorts and keep yourself from being so easy to grapple.
All in all, I think grease is a pretty great spell to have on hand. Usually the only time I think it is NOT a viable tactic is when it is going to anger your melee teammates by ruining their effectiveness.
