I originally bought this because it was only a tiny bit more than Diaspora alone, so if I liked it then I'd have another game with the same basic rule set, and if I hated it I only paid about $2 more (at the time I bought it) than I would have paid just for Diaspora.
So far I haven't even looked at Spirit, and I'm impressed. I've read some other FATE based systems, so I had an idea of what to expect, but the detail for different areas from Ship combat to Social combat is exceptionally well done. The example from the play test for using social combat to influence a planet on page 169 shows something the vast majority of other Role Playing Games don't even try to touch, or if they do, it so simplistic as to be boring (a simple skill check or something). The only other game I've seen really try to tackle social combat is Exalted, and while not terrible, it isn't as fun in play as I would have liked.
Once you know the basic system, understanding the small changes for the other mini-games is pretty easy, and the character creation helps build characters with a sense of history and connection to the other player characters, which I find really helpful. Though adding a new character (for a new player for example) to a later session requires a little thought as it isn't specifically mentioned.
My only gripe is I'd love a mobi formatted version for my kindle as the areas with side bars don't convert correctly and become very hard to read through very conversion process I've tried so far (and native pdf viewing isn't all that great on the Kindle in my experience).