I got all 150. IT IS A PIECE OF CRAP. Nothing good happens at the end.
WRONG. It unlocks the cannon outside the castle...but I'll admit it's rather anticlimactic.
Yeah, the characters aren't balanced that well. Luigi's spin tornado, for example, enables me to get some stars that are supposed to be only accessible by Mario (i.e. "Mario's Super Wall Kick" in Cool Cool Mountain. Also, there's no incentive to play as Wario unless you have no choice, because he is so freakin' underpowered in every area other than strength.
On an unrelated note, check out my coin highscores on the levels (No hacking was involved...although a few of them took advantage of opportune glitches. And with the exception of Big Boo's Haunt, all of these scores were set while playing as Yoshi).
Bob-Omb Battlefield-------182 Coins
Whomp's Fortress---------205 Coins
Jolly Roger Bay-----------135 Coins
Cool, Cool Mountain-------255 Coins
Big Boo's Haunt-----------160 Coins
Hazy Maze Cave----------176 Coins
Lethal Lava Land---------214 Coins
Shifting Sand Land--------167 Coins
Dire, Dire Docks----------147 Coins
Snowman's Land----------255 Coins
Wet-Dry World-----------157 Coins
Tall, Tall Mountain--------197 Coins
Tiny-Huge Island---------255 Coins
Tick Tock Clock----------132 Coins
Rainbow Ride-------------255 Coins
Note: It is physically impossible to get more than 255 coins. Once you hit 255, you can still collect coins, but they are not added to your score.
Now, here are my scores on the minigames:
Wanted!--------------------256 Stars
Loves Me...?----------------N/A
Hide and Boo Seek-----------8 Stars (I'm not good at this one...)
Puzzle Panel-----------------12 Stars
Boom Box-------------------17 Stars
Tox Box Shuffle--------------13 Stars
Which Wiggler?--------------15 Stars
Mix-a-Mug------------------1,057 Stars (That's no joke! Took me a while.)
Puzzle Panic-----------------12 Stars
Mario's Slides----------------33 Stars
Bounce and Pounce----------54 Stars
Sort or 'Splode---------------193 Points
Trampoline Time-------------126 Points
Shuffle Shell-----------------2,000 Points
Bounce and Trounce---------234 Points
Connect the Characters------23 Stars
Shell Smash-----------------64,660 Points
Trampoline Terror------------41,800 Points
Memory Match---------------Currently 57 Coins
Pair-a-Gone------------------Currently 89 Coins
Picture Poker-----------------Currently 1,737 Coins
Mushroom Roulette-----------Currently 84 Coins
Mario Slot--------------------Currently 82 Coins
Lucky Stars------------------Currently 48 Coins
Pair-a-Gone and On----------520 Points
Memory Master---------------Currently 35 Coins
Super Mario Slot-------------Currently 3,516 Coins
Bob-Omb Squad--------------106,200 Points
Snowball Slalom--------------13.53 Seconds
Bingo Ball--------------------20,900 Points (a perfect score!)
Coincentration---------------36 Points
Psyche Out!------------------21 Stars
Slots Shot--------------------51,800 Points
Lakitu Launch-----------------28 Points
Intense Coincentration--------16 Points (a perfect score!)
Giant Snowball Slalom---------26.53 Seconds
What are your scores?