Let's face it, it's a good pastime. The main mode, tournament, is nice and well structured, with tournaments increasing in level, but not very practical because it can take a while and you can't save progress, but it's still a lot of fun.
The character selection is good, as always with Peach serving cunt. The design is very fun and even attractive, it awakens an inner child that makes you want to play.
The ring mode, where there are rings on the field and you have to cross them to score points, is difficult and a bit confusing, but it's still a good mode; the piranha mode, where piranha plants spit balls at you and you have to hit them back, is the coolest of all and the most enjoyable for me; the bowser mode is interesting, but a bit boring, because it's just the usual mode with power-ups that do practically nothing.

On the surface it's a simple tennis game with Mario characters, and that's exactly what it is but there's just something about it. The controls have aged remarkably well. They are easy to learn, hard to master which is what most games should strive for.

If you have a friend I recommend trying this out with them. When I first got my NSO+, we went through a bunch of games and while we kinda picked this one just for the hell of it, we kept playing and now it's basically tradition to play this game at least once when we meet up at this point.

I'd recommend playing doubles, it really is the most balanced and skilled way to play this game. For me, I really clicked with the way Paratroopa plays. He's speedy and can really slice through cracks in the opposition's defences.

Honestly just try this game. It's way more fun then you'd first expect.