Absolutely amazing. Really a shame I was scared off of playing 4e back in the day by a bad launch. This standardizes terrain effects in a way that really encourages PCs to examine their environment and just generally makes the map more interesting.

The gist of this supplement for D&D 4E has some nice features for designing new terrain features for your campaign. Sure, the standard core books have guidelines, but there are some handy charts here to scale damage without having to think about it, etc. It leads you through some choices to determine if the terrain is activated, moves, changes, morphs, etc. and then offers suggestions on how you might implement these features. Of course, you could just take the easy way out an use one of the pregenerated examples (of which there are 20 pages worth, even organized by type in an index).

It's a nice supplement for using some cool terrain in your games, which always makes things a bit more interesting.