Just finished today (as I type the end credits are playing). I have to say that I enjoyed the game more than either the M&L or Paper Mario games. The pacing is just perfect, if a little easy. I finished it without any grinding (even beat Culex) though the Lazy Shells helped a ton. In fact right around the sunken ship I started avoiding every enemy I could.
I love the M&L games, especially Superstar Saga, and generally despise the Paper Mario games because of the pacing problems and extensive dialogue that is not as funny as they think it is (I can pick up Metal Gear for that, thanks). But neither of those series capture the pure strangeness and fast pace of this game.
There is a Luigi cameo, BTW, but I won't give away where.
Thanks for getting me to play it again, it was completely worth it!