Overall, Sega Superstars Tennis is not a great tennis game, but it's certainly not a bad one, either. It looks pretty, plays well (if simply) and runs great. It also has 16 of Sega's best characters, though, in practice, the game doesn't have that cartoonish, wild-and-wacky feel that other animated sports games (namely, Mario Strikers) have.
It's basically Virtua Tennis, except that character building and real players have been replaced by a healthy dose of nostalgic fun. Worthwhile for a weekend, to be sure, but unless you're a die-hard Sega fanatic, there's not much long term fun to be had.