What is so good about Grease?

[Epimetheus]

So I've always wondered. Does Grease stack with Plant Growth? Speed drops to 5' from Plant Growth (no save). Does it drops in half again to 2.5' from Grease. That's effectively 0... Then again, Plant Growth seems pretty cheap to me if your team consists of caster blasters. (Or flyers)

Druids and Rangers that cast Plant Growth enjoy being able to move unhindered while everyone else is slow (they both get Woodland Stride before they can cast the spell).  Honestly I'm not sure if the movement penalties would stack, but it's worth considering.

Druids get Woodland Stride, which would fix things... assuming they didn't prestige out of Druid too quickly. (heh... oops).

[RobbyPants]

It's not spectacular, but it's about the best battlefield control you'll get out of a first-level slot, especially a core-only first-level slot. Color Spray or Sleep is better very early on, but Grease retains at least niche applications after those have stopped being usable.

It can remain useful at higher levels because the best effects of it are keyed off of ranks in Balance (which tend not to scale with printed monsters) rather than the Reflex save (which does scale).

So, you run into a CR 15 monster, and there's still a really good chance it still has 0 Balance ranks.  About the only thing that makes it less useful is that you're more likely to run into fliers, incorporeal monsters, or guys who can otherwise ignore it as you get to higher levels.