It's a fun little distraction... but not enough staying power to make it worth a purchase. Decent rental.
The control scheme isn't quite like Wii Sports Tennis, which may lead to some dissapointment, but I didn't have a problem with it. Rockstar went with a slightly more strategic and definately more realistic approach with this one. While you can swing at any point while the ball is approaching you, making timing less of a critical issue, the game relies more on the spin and aiming aspects. This "lag" between controller swing and in-game swing seems jarring at first, but you realize it serves it's purpose well. The animations are spot on and you can quickly get lost in how cool the whole thing looks. Sure, it's not a direct feedback game like Wii Tennis, but come on, that game doesn't have to animate realistic player models. The control scheme works well for what it sets out to do. Overall it offers a much deeper, if somewhat less immediate, gameplay mechanic than Wii Tennis.
On the high difficultly setting, games against the CPU can get pretty intense and it's a rewarding game to work your way through. Unfortunately, it doesn't take long for you to get there. This one just can't be recommended for purchase based on this limited staying power. While enjoyable, it's not something that most people would revisit after a weekend or so spent unlocking everything... and Wii Sports Tennis is always going to be preferred when 2-3 other people are hanging out.
I'm giving it a 7.0... which is slanted towards rental. If looking at this purely from a purchase perspective, it's probably a 5.5 or 6.0 due to lack of staying power.