Leto's in many ways the most human of all the characters in the series, perhaps in all of literature IMO. Ironic since he's physically an inhuman monster.
The last two books are interesting because in many ways they go more towards traditional scifi - space battles, battlecruisers, etc. Society also becomes a lot more like modern society (again). It's interesting to look at the universe throughout the books - in Dune, we're introduced to a feudal society, in Messiah the society's evolved into an absolutist empire, by GEoD the empire is hypotheocratic, and then a few thousand years later, we have essentially modern society except some 13000 years in the future. It's probably best demonstrated by how Giedi Prime has evolved since we see more of it than of any other planets (save Chapterhouse but that's a bit special) with Leto acting as something of a Christ figure.
Even though the last two books are my least favourite of the series, I still love them because of how wonderfully Herbert evolves his society and culture over the 5000+ years that the books cover.