I have to admit, I wasn't expecting what this game turned out to be. It's very different to the original Goldeneye in terms of the layout of levels and the types of weapons you get, but after a while you realise that the style of play is not too dissimilar. One thing that I love about the game is that the stealth aspect is really well done. You can either approach a level in an all-guns-blazing fashion, or see how far you can get by sneaking around, taking bad guys out with your silenced PP7 (although headshots are a bit temperamental - a very fine line between a satisfying one-hit wonder and having to follow your first shot up with 3 or 4 more before he raises the alarm) or a stealthy melee attack.
The single player is good fun - I have finished it on Operative level and there is nothing too challenging about it. I'm about to start the next level up, which I hope will be a bit harder.
The online multiplayer is really the highlight - all-new levels to blast your way around, either in your own or as a team. As mentioned in another post below, the game throws you into a game with people who could be 30 levels higher than you, so it can be a little frustrating at first, when the seasoned players are taking you out every 10 seconds. AND it takes ages to earn any decent weapons in the levelling up process, which doesn't help the novice multiplayer.
In summary: great single player, great multiplayer - best shooter on the Wii - although both take a little while to get into. Definitely worth getting.