The most important things I've found that make Google Collections the place to start:
- Generics (Collections without Generics -- FTL)
- Consistency with Collections framework (Josh Bloch was a key player in this framework)
- Correctness. These guys are desperately tied to getting this problem right; they have something like 25K unit tests, and are tied to getting the API just right.
Here's a great Youtube video of a talk that was given by the primary author and he does a good job of discussing what is worth knowing about this library.
