If you're lightning-quick and a Mario megafan, it's good fun. For everybody else, the cruel frustrations will NEVER end.
Deaths in this game are so cheap that it's laughable. For example, if you land on an enemy's head, you'll usually defeat it, but even if you just miss the top by a fraction, it counts as a death. There are also many points where you have to cross a certain distance, but enemies can get in the way or the platform you're standing on is awkward and throws you off too easily. 1-hit deaths means frequently having to replay levels and waste power-ups. Oh, and if you lose all your lives, the game undoes all of your progress in a particular world - everything you worked hard to get will have to be got again. When you play two-player, arguments can easily start from Mario and Luigi failing to co-operate because of the crowds of clumsy enemies and wonky ledges. Controls can be unresponsive and can spoil climactic sections of levels.
There are plenty more annoyances.
1. When you get a power-up it runs away from you, and it'll usually run into a dangerous area where you can't get it or off the edge of the level.
2. Getting star coins is far too tough. They're usually in places littered with enemies, hidden away in cheap, dangerous places or put so high up in the air you have to find some way to climb up and reach them.
3. You'll often find power-ups you don't need in levels. Why can't you store these for later for when you really need them? Why must you use them immeadiately? This would make levels a lot easier to get through.
4.The game often forces you into some dull but dangerous minigame when you cross the map to get to a level marker. Your only reward for these minigames is mushrooms, and the only way to quit the minigame is to sacrifice a life.
5. The physics in this game are ridiculous. For example. Koopa shells travel uphill at exactly the same speed as when they go down a hill. Why can't Mario do this? Another physic problem is Mario's ability to stand on a platform. Sometimes he'll jump onto one fine, with no problem. Other times you'll jump right onto one and then slide right of the edge as if it's made of ice!
6. Rescuing Toad from hard levels is also not fun, and wastes time.
The game isn't entirely bad. The graphics are wonderful and the sounds and the brilliant music are like classic Mario. Plus, there's some good rewards and finishing a level is usually rewarding. It's just the fact that the difficulty is so high that frustrations makes the rewards not really worth it. Another good thing is that lives are quite common, so at least you get lots of attempts - even if lives go down the drain in seconds.
One of the best parts of the game is riding Yoshi though. Yoshi is always great fun and can generate items for you (wish you could store them for later though). Sadly, Yoshi isn't in it much.
But I found it far too hard to enjoy it, and it's a real shame because I really wanted to play a good Mario game that's hard but fun. This game is annoyingly overrated. Don't believe the people who call it the best game ever, those people are megafans that can avoid all danger so they don't know how hard it is for beginners or less-good players.
From my POV, Bowser's Inside Story, Mario Kart Wii and the two Super Mario Galaxy games are much better options.