I've only just begun, but the game so far is aces. The writing is excellent as expected, with Bowser's and Fawful's dialogue being the best of the bunch. The beginning sequences are extremely tutorial heavy, though; like the first two games, they'll cover everything from overworld movement to battle commands. They can be skipped, but they're worth sitting through to experience the cleverly wordy exchanges amongst characters.
There's also a much welcomed graphical face lift; further animation has been given to the characters, but more significantly, the fonts and menu screens got a total revamp, resulting in a sleeker new look. There also appears to be a new sound font set in place, so even standard sound effects like hammer hits and voice overs are refreshingly different. All this makes the game feel like a much more nominal sequel to Superstar Saga than Partners In Time was.
The Mario/Luigi portions have been enjoyable. I dig how these sections are played out in traditional 2D plane style, so you've got a definite Paper Mario vibe going on here. Bowser's is fun too. The big guy doesn't jump, so you need not worry about having to fumble around with dual-leaping platforms (one of the main faults I had with the previous two games). His appears to be more puzzle oriented, where you use your two primary "smash" and "fire" button techniques to get through.
And Shimomura's music score is terrific as usual. Especially the battle themes; Bowser's Battle Theme, in particular, is quite excellent. And those opening bars to the end of battle Fanfare theme are way infectious.