The game is definitely worth getting. While I have only reached the fifth world (of eight) it's fair to say I've seen enough of the gameplay to form a general impression of the game. It is a 2D sidescroller in the tradition of the older Super Mario Bros., before the 3D "turn" on the N64. In fact, a lot of the levels' themes will remind you of levels in these older games. Mario still has his standard moves and not much more: jump, slide, spin, wall jump, etc. In addition, there are the ice and fire powerups, and there are two new "suits": one where Mario can transform into a helicopter allowing him to span the entire screen with a simple jump and spin of the Wii remote; and another where Mario turns into a Penguin, allowing him to belly slide over water, swim better, and shoot ice balls (like the ice Mario powerup). The worlds follow your standard themes: a water world, an ice world, a jungle world, a lava world, etc.
Sounds pretty standard for a Mario game, right? The big innovation that will really draw people to the game and enhance the experience is the cooperative mode. Basically, one can either move through the game according to the standard 1-player format (complete it 100%), or play the levels with up to three other characters on the screen at once. This added dimension really makes the game a lot of fun, though it is not without its weak points.
In terms of positive aspects, the cooperative mode adds a real feeling of teamwork and team accomplishment as you go through the levels. As anyone who has played one of these sidescrolling platformers before knows, they can be very, very frustrating experiences. But at the same time, pulling off a difficult maneuver, especially with another person there to cheer you on (and maybe even help you!) is a great feeling. Sometimes teammates will want to communicate with each other in order to get an optional coin (of course, with a bonus attached) or take care of a particularly difficult sequence of platform jumps. There is also a specific set of built-in team abilities, such as lifting characters to throw them to locations, or taking advantage of the helicopter ability mentioned above that are really fun and occasionally useful.
On the negative side, there are the obvious issues with having one or two good players and then a third player who isn't as good that can definitely affect the experience. This normally comes into play when the third not-so-good player gets in the way on platforms or can't execute a team strategy. This of course could affect any cooperative game so it's not really a levelling criticism. What really seems to be a weakness is that while the game is meant to have four players on the screen at once, there's really not much need for more than two -- the team problems you have to solve, as well as your ability to navigate the screen, seem most effectively handled with two players! The only reason I can see myself being wrong on this is because I've only had one other "interested" gamer to play with (one of my brothers) in the sense that my other brother and my sister either don't like it that much or aren't that good. Perhaps if I had two other people who were really interested the experience would change, but I'm skeptical at this point.
Here is a fuller review you may want to check out: Gamespot has the advantage of historical knowledge and says this game is one of the harder Mario's to ever come out, giving it an 8.5 based on this difficulty and the richness of the gameplay/replayability but also partly agreeing with me over the ambiguous nature of the multiplayer.
Overall, however, I really do think it's worth it and definitely whets my appetite for Super Mario Galaxy 2 in the first half of next year!