In particular, I like how extensible it is. It has let me add my customizations to the druid spell list (and I was able to buy the additions for Masters of the Wild for $2.50), and add Liandra's proficiency with a bow. I could add a definition of the rat-kin race without too much trouble, and have the bonuses be well-integrated. And for really quirky things like the chalice of healing, it will allow me to just assign a name, a weight, a cost, and a description of its effects. I highly appreciate this extensibility--since I do come up with wacky ideas and variant rules, it's something I really need in a character generator.
It's also proved very useful for creating monsters. It's very easy to choose a monster race, advance it a bit, add some templates, and whatnot. This will let me use a lot more advanced monsters in my game.
Down sides: it's open-source nature is showing. The installation wasn't too bad once I understood how to do it--but that information was hidden in the last pages of the documentation. (The answer: unzip all three zip files into the same directory. Use an empty directory, not one with a previous install.) By the same token, the user interface is a bit crufty in places--it seems somewhat characteristic of open-source.
PCGen is available at http://pcgen.sourceforge.net.