I think I prefer Mario & Luigi to being limited to just the Bros. It works out well. The battle system fits it perfectly (especially with the Bros. attacks). You don't have to worry about buying equipment for tons characters and then organizing them into a small party to decide which characters will work best in what dungeon. Not to mention extra characters would detract from the coolness of a lot of the dungeon areas. I especially enjoyed Joke's End, that was a great one.
But the game is just fantastic. The dungeons are well designed, the battle system is actually fun, the writing is great (Fawful is the king - "Princess Peach’s voice will soon be the bread that makes the sandich of Cackletta’s desires! And the battle shall be the delicious mustard on that bread! The mustard of your doom!"), the soundtrack fits perfectly, and you get to play as Luigi! Luigi rules.
Seriously though, the mustard thing rules, just like Hostile said.