Mario Tennis Open's graphics and multiplayer modes don't make up for the lack of meaningful singleplayer play.
The game is similar in its approach as you'd expect it to be. There are a variety of Mario characters playing tennis. Where the game instantly falls short is its difficulty level. The game is boringly easy through the first ten hours of gameplay, and as soon as the game begins to pick up in difficulty you are already feeling done with it. They also have added a "simple shot" button (X button, as well as an area of the touch screen). This button essentially picks whatever shot is best for that situation, effectively allowing someone who is braindead to beat the game.
The game lacks anything to keep you coming back to it. Unlike some of the previous handheld versions, there is no plot to it at all. There are four characters to unlock in minigames, though if you've played any game in the franchise before you should get them all on roughly your first try.
The multiplayer makes the game somewhat worth it. Playing against something with human level intelligence can occasionally make up for the lack of any sort of AI challenge. As well, the graphics are beautiful. It is hard to believe it is a handheld game at times.
Overall, the game disappointed me by having an incredibly short life with very little to do outside of exhibition matches, and being way too easy. I think this may be a series that has been hurt by having more controls available. Anyone who played on the N64 or gameboy will remember the satisfaction of double-tapping A and B to create a smash shot. Now with so many buttons available on the 3DS, the smash shot has been reduced to a tap of the stylus or the Y button.
In the end, it is a worthwhile purchase if you are looking for a game to pick up and play for 10 to 30 minutes at a time to waste some time. That being said I would not buy it for the asking price of 40+ dollars.