Just installed it.
the interface takes getting used to - it's column-based and very effective, though.
You get complete dictionary definitions underneath the browsing colums of whatever word you click on, and the next column on shows folders of synonyms by "more general", "more specific", "similar", "opposite", etc.
Each folder lists the appropriate terms, all defined, and each with an identical list of subfolders, and so on.
System-wide thesaurus is installed in the Services submenu, as well, though the key combos (Cmd-< and Cmd->) aren't working for me.
And it's free as in beer.
I've wanted something just like it for the longest time.