I enjoyed unlocking all the possible titans and their upgrades. The little descriptive notes about their backstories were a nice touch. It's fun to think of your character as a centuries-old, near-mythical powerhouse.
It was hilarious to see the thoughts of the human soldiers. "Are you nuts? It's a dumb beast, it can't understand us! More tanks!"
The upgrades appear as visual effects on the Titan's body, which is great. It was very satisfying to watch my initially unimpressive Torugon transform into a powerhouse.
--For a beginner like me, who tries to power through the levels on brute strength and endurance, the Giburas and Antras are pretty much useless. For example, the Giburas's non-upgraded melee attacks don't seem to do the same amount of damage as, say, the Gomorra's non-upgraded melee attacks. And although I've got the Antras fully upgraded, I can't get it past the second level.
--Half the fun is finding what sort of new Titan feature you'll unlock. But the character designs are all fairly similar: the Madoka looks an awful lot like a purple Antras, and the Gomorra and Garubon are both quadrupeds with tail-whip attacks. If you're taking inspiration from the Kaiju--think of Rodan, who was a fast flyer, or Ghiborah, who had several heads.
--Perhaps I'm just not very good at the game, but I find it a tad tricky to juggle the ranged and melee attacks while moving my titan--and the ranged attacks don't seem to do all that much. It would have been much more satisfying if the focus was just on melee and stomping.