When I first saw this title, I was skeptical - "Isn't this basically a Sega version of Mario Tennis?" While it doesn't exceed that expectation or comparison, Sega Superstars Tennis is still a better game by itself than I expected.
Unlike Mario Tennis, which only uses characters from the Mario franchise, Sega Superstars Tennis is more like "Super Smash Brothers" with tennis. Characters from almost all of Sega's series are here, with 8 in the beginning and 8 more that can be unlocked by playing the game. The series represented are Sonic the Hedgehog, Jet Set Radio, Samba de Amigo, NiGHTS, Space Channel 5, Super Monkey Ball, and even classics like Alex Kidd and Golden Axe. There are characters from each series (5 characters from Sonic, while most of the other series have 1 or 2 characters each) as well as a themed stage. Furthermore, some series that didn't get characters in the game still have stages, such as House of the Dead, Puyo Pop, and Virtua Cop. Basically, if you've ever owned a Sega Dreamcast, there will be at least one series in this game that you will enjoy.
The gameplay feels a little more realistic than Mario Tennis; the only "gimmick" is the Superstar Mode that each character charges up by hitting the ball back and forth and getting points. Once the Superstar Mode gauge is filled (represented by a star underneath the character going completely golden) the character can activate Superstar Mode. The main effect of Superstar Mode is to change the pattern that the ball is launched in. It's different for each characters - some characters cause the ball to spiral around, some cause the ball to zig-zag, and some cause the ball to make sharp, sudden turns. There's usually another effect as well, some sort of obstacle launched at the enemy to make it more difficult to hit the ball. For example, Dr. Eggman's SS Mode launches waves of electric mines with each hit, and the opponent has to dodge those in addition to hitting the ball. Using Superstar Mode makes it more difficult for the enemy to return the serve, but it's by no means impossible once you get the patterns down, which is nice.
Other than that, the game is pretty simple tennis. There's no "charge levels" like in Mario, and there's no items or anything on the field either. The characters handle noticeably different from each other; characters like Beat from Jet Set Radio move really fast but don't hit hard, and when you switch to a heavy character like Dr. Eggman, it's easy to notice the difference in power and speed. However, the game also feels sort of empty because of this simplicity, as well; it's just tennis that happens to have Sega characters playing it.
To offset this, there are some thematic minigames (involving tennis) that can be played on some stages. The House of the Dead stage has a minigame where you need to avoid zombies and knock them down with the tennis balls that a machine is launching at you. Jet Set Radio's stage has you collecting spray cans lying around the court. These minigames use the same controls as the main tennis game and provide a reasonably interesting diversion from the main game.
The graphics and sound are both pretty good; there's a diverse selection of music for each stage, and both the stages and characters are rendered reasonably well. There's some voice acting in the game, but it's limited to grunts and post-game taunts. Neither is worth getting the game for by itself, but it helps make the package more solid.
Probably the main problem with this game is that despite the array of characters, stages, and minigames, it doesn't feel "Sega" enough. There's a good array of games represented, but the gameplay itself is basically Tennis with a single charge-up gimmick. Whether or not that's good or bad is up to you, but to me personally it didn't feel like there was enough character in each match. However, as a whole the game is pretty good, and worth it for a fan of any of Sega's series.