I don't agree with the people whinging about the controls sucking, people are expecting the wrong thing with this game if they think that.
It ISN'T supposed to be like Wii Sports, people have got to stop thinking that for starters. It is more in line with Mario Tennis/Virtua Tennis than even being close to Wii Sports in the way it plays. The game is built for you to control your own character, that's where the skill and depth is.
CONTROLLER OPTIONS
Wiimote Only: The wiimote only option, allows you to let the CPU take control of your character, while you swing when needed. "Similiar" to Wii Sports, only Wii Sports has swing mechanic, that while no where near perfect, is much more timing and swing based than Sonics, which uses the Wiimote only in a "timing your shot" way. If you play the game this way, Wii Sports is the better game, BUT, you are also missing out on how this game should be played.
Wiimote and Nunchuk: How the game should be played, by anyone giving it a go. Control the character with the Nunchuk, swing with the wiimote to hit, (only timing based) and direct your shots with the Nunchuks analog as well. You can do lobs while holding down A, or go for drop shots while holding down B (need to swing for both too). Using this control scheme, Sega Tennis plays like a "proper" game of videogame tennis, in the same way Mario Tennis, Virtua Tennis, Top Spin, Table Tennis have in the past.
Traditional: Use the Wiimote, utilizing the digital pad to control your toon, while using the 1 and 2 buttons for power shots, slow shots, lobs and drop shots.
All characters also have a power play that builds up as you go through shots etc.
IMO, everyone is jumping into this game, thinking it is trying to Mimic what Wii Sports did with its control scheme, when the exact opposite is true. If anything, as long as you use the combo setup, its really just replacing the "swing button" with swinging the wiimote. Reason the 360 game plays exactly the same and is also a heap of fun against human opponents.