First of all, I'd like to say that I've had a lot of fun with this game. The cheesy and simplistic gameplay grabbed me and it held my attention for a long time. Hardcore tennis fans or gamers would probably find this game very tedious and perhaps too simplistic for them. As mentioned in the summary, the achievements for this game can be a pain to get, especially in multiplayer. Now let's get down to the details.
Pro Tour mode is quite a bit of fun, but isn't for everyone. You can Create-A-Character and make him/her look however you'd like. As you win money during this mode, you can upgrade your gear and make your player unbeatable. Throughout the season, you can play in tournaments, rest, or play special matches. Tournaments will increase your ranking/skill, resting will increase your stamina, and special matches will increase your popularity/skill. During the first few years of career mode, beating the top opponents can be very challenging at times. Nevertheless, once your character nears level 100, the competition becomes very weak and you can usually win every match in a year. You can also simulate matches in tournaments and makes the process of winning tournaments easier.
Exhibition mode is pretty much your basic exhibition mode. You can choose a player (including the ones from your careers) to play any other character in the game. One downfall here is that my level 90 character can beat my level 100 character on the hardest difficulty setting. It works out the same way vice versa, so it's just a bit too easy.
Online is pretty unplayable and the servers don't always work up to expectation. Also, there are very few players that play this game online anymore. Games are very laggy and matches usually disconnect about 20% of the time either due to a bad server or poor connection quality of the other player. Most of the online achievements are difficult because they aren't necessarily clear cut in describing how to achieve them (see the guides of those achievements for more details).
All in all, it can be a pretty fun or addicting game for the right person. To others, it may be annoying or downright irritating. Happy gaming!