Best game on the Wii. In the top 10 games ever.
Gameplay is pretty much the same as Mario 64 - you'll be jumping around a variety of different levels, collecting coins and the all-new star bits, which act as both a weapon and as currency to unlock a few hard levels. However the new space theme adds a new gravity mechanic which is just great. The space theme also adds Lumas. These are little star people which turn into launch stars, which allows you to shoot from one strange spherical object to another. There are a bunch of other space themed mechanics which all work really well.
The level design is great. Levels can include a bunch of spherical objects that just float there in space or the sky. You can usually walk on top AND bottom of these. Other parts of the levels are normal Mario staples with the same mechanics (though some are just normal 3d platforming )- you have your ice worlds, sand worlds, fire worlds, underwater worlds and just normal grassy environments. The camera copes very well with all the turns especially on the space levels where you will go upside down. When it does go wierd though it is really annoying. You can sort of adjust it. You can get a bunch of suits such as fire, ice and flight. They all work well apart from the hard to control spring powerup. This turns Mario into, as you would expect from a powerup called the spring, a large turkey drumstick. I mean spring. Maybe it's just how well Mario controls normally and with all the other suits, but the spring is just really annoying expecially on small platforms, and you will frequently fall off the level into black holes, which crush Mario like the puny Italian plumber he really is. Then they spew him back up to the last platform, in exchange for a small picture of Mario ('lives' in the game). The suits overall add to the level they are suited in (you won't find a fire powerup in an enviroment that doesn't suit it... usually *hint*).
You access levels from a hub which has several small hut-like areas in them. There are huts like the garden (because everyone knows gardens grow in space inside a small building) and the bedroom, kitchen etc. There's just over 5 of these and every one has about 3-4 normal levels in them, 1-2 bonus challenge ones and a boss level. In the normal levels there are themes like the aformentioned ice, fire, space and grassy enviroments. These play pretty much normally, and you can get about 6 stars - 3 normal ones, a hidden one and 2 'comet' ones, The bonus, challenge type ones are more strange and abstract, and usually give you one star. The boss levels are either against Bowser/Bowser Jr. and they work great, and they are sort of like Mario 64 (but no bombs this time). Obviously the number of stars and kind of levels vary. The way the system works gives you a lot of variety and choice - you can stay in the same level and get all the stars or you can just get enough to unlock the next ones. Comets sometimes enter levels. These add new aspects to areas in the level, for instance you can get speed run and dare-devil (one hit and you die) comets. These are pretty cool and challenging. Your goal is to get 60 stars then beat Bowser. Then there are another 60 optional stars. You can get all 120 stars, get a bonus, then get the same 120 stars for another bonus if you really want to. Some of the extra 60 stars will be levels you just haven't done, but the rest will be either hidden, comets, or the one problem with the game, purple coin levels.
In purple coin levels, you are given a version of alevel where there are 100 purple coins in it. There was no reason to include this and you will enevitably sit there looking for those one and two ones that they've hidden away somewhere. It is really annoying and some are harder than others (eg. the ones with more fiddly hidden places in them).
The graphics are fantastic and the best on the Wii. There are a lot of effects like the lava, ice and water, which look REALLY nice. You aren't gonna get Crysis but the whole design of the levels look great. The Gamespot review by Alex Navarro says that there are rare framerate problems - they must be really really rare because it never faultered for me.
The sound is just as good. The soundtrack includes old favourites and others which you haven't heard yet - but they will also be your favourites. The music is all orchestrated and whoever decided on the orchestra decided right. It all fits perfectly with the levels. The sound effects are great. Collecting coins, star bits and crushing innocent creatures skulls all sounds great. Unfortunately when you long jump Mario into a brick wall he doesn't lie on the floor screaming in agony with blood pouring out of his eyes, he just utters a cute "ooh!". Thats alright though. The small amount of voice work is good and fits the characters. It's hard to describe everything in Mario Galaxy because it does vary a lot. But it has a fantastic charm to it and the graphics, gameplay and audio are the best your little white box can get.