Mario Power Tennis is easily the best Mario sports game ever made.
No Mario sports game is perfect, but Mario Power Tennis does a pretty good job with about all of it's categories. My gripes are that no matter what difficulty level your opponent is on, they will be easy. You can select Expert difficulty and take them just like you would on easy in some other tennis game. Also, the single player experience isn't as long and it would've been cool to have what Camelot did to the Gameboy versions of Mario Tennis-A tennis RPG game. The single player mode gets harder, but it's actually not all that hard. When you get to later matches you'll find them easy but they will take a while on. Multiplayer is where this game deserves respect for. The fou rplayer matches are great, and it is splitscreen unlike in Mario Golf: Toadstool Tour where you go by turns.
Another highlight is the minigames. There is a variety of tennis-based minigames what involve you doing tasks like hitting ghosts with tennis balls, playing tennis with a giant squid, and a lot more minigames. Some of these have multiplayer, and the minigames are very fun to play. There are also unlockable courts, characters, and more.
The graphics are standard for what you see in other Mario sports games. If you've played Mario Golf: Toadstool Tour then you will know what the graphics are like. There are pretty good and very colorful but they are not quite the best. Not really even that close to being the best but a las, they are a vast improvement over Mario Tennis for the Nintendo 64.
Just like in previous Mario Tennis games and other Mario sports games, most of the music is remixed versions of songs from previous non-sport Mario games. You'll have music in Power Tennis from the original Super Mario Bros, Super Mario Sunshine and a wide variety of other Mario games out there. Most of the voices are a guy who will say things like "match point" or other tennis lingo along with a few characters who talk occasionally. It's a tennis game so it's really not going to have too much of a dialogue.
So, is Mario Power Tennis worth forty to fifty dollars? I say that it is because the game will keep you hooked on it for a while. However, if you live out in the styx and have no one to play it with than it is not worth it because the highlight of this game is it's great multiplayer which will keep you playing for months.