It's fun, but in hindsight, it's insubstantial
You've probably played this game. Even if you've somehow managed to avoid picking up a Wii all these years, you've undoubtedly seen it at a friend's house or at a store display. Wii Sports is one of those cultural phenomenons that will probably be remembered for decades to come- where kitschy colors & hair metal represent the 80s and hip-hop and The Simpsons represent the 90s, Wii Sports will one day come to represent the 00s. It was just that big.
"Big", that is, in the sense of popularity. In the sense of depth, there really isn't much to actually play in Wii Sports. There are only 5 sports, and the majority of them have been simplified quite a bit. That's not to say you won't have fun, but you'll definitely be begging for more by the time you've explored it all.
The first sport, and easily the most basic, is Tennis. The only input necessary is swinging your arm; all movement is done automatically by the Miis. In single-player and two-player, the player controls both characters on their side simultaneously, and though it doesn't take away from the gameplay, it's quite amusing to see one character aim at the ball while the other flails at nothing. Really, this is one game that's better off with friends. With or without them, it gets extremely tedious after only a few rounds, but at least with them, you're socializing.
The second sport is Baseball, and once again, it's been heavily watered down. The only movement is pitching and hitting; running is, again, automatic. This minigame is probably the most dull. Hitting/throwing a ball over and over really isn't that much fun. In simplifying baseball, Wii Sports also robs it of everything that makes it enjoyable- and it doesn't help that this sport can only have 2 players.
The third sport is Bowling. This is the big one that everyone's heard of: the talk of the retirement homes, and the life of every party. Surprisingly, it's just as engaging as advertised! I don't quite know how bowling is so fun (it's just rolling a ball down a lane forever and ever...) but in some primal way, it is. The nice thing is, this minigame is basically the real sport, with no simplifications or limitations. Bowling's controls are occasionally a bit spotty - move your arm the wrong way, and the ball will somehow go flying behind you - but it's a small price to pay for a great experience.
Sport number four is Golf, without a doubt the worst of the entire package! It's a crying shame, because, like Bowling, this minigame actually does a good job of simulating the real sport- you can swap between several different clubs, fine-tune your Mii's position in a variety of ways, and admire the genuine serenity and beauty of the levels' landscapes. But accuracy is no substitute for playability, and Golf is almost unplayable! The controls are absolutely impossible to pin down, as the Wiimote seems completely unable to respond to minor movements, and large movements are constantly misjudged. I've been in situations where the ball is right next to the hole, and I try to do a slight flick to plop it in, but the game doesn't even register it. So I increase my power very slightly, and suddenly the ball flies 100 feet away from me! So after several more strokes I finally get the ball back to that position again, and again try the very slight flick... and this time, it works! A VERY inconsistent minigame indeed.
The final sport, and my personal favorite, is boxing. What a great little game this one is! The controls work great, and you always feel in perfect control of your little Mii boxer. It's quite basic, but the good kind of basic (and it helps that real boxing is a pretty simple sport already), and it's noteworthy as being the only minigame in the package that is just as fun to play alone as it is with others. Bashing away at your friends or the computer is untiringly fun, and the most impressive part is the difficulty curve: CPU boxers get increasingly, but very gradually, difficult as you continue playing. To this day, I still find myself migrating back to this game to challenge the next-tiered boxer.
Bonus modes are slim pickings. There are two of them: "Training" and "Fitness". Training mode is pretty fun, providing you with 15 minigames based on the 5 sports (3 for each). Hitting a lot of balls in a row, or smashing away at punching bag after punching bag, is a blast. The "Fitness" tracker, however, could barely be called that: it's just the Training minigames moved around in a random order. The game doesn't really track how fit you are at all, just how proficient you are at the minigames.
Wii Sports is a tiny game, and isn't likely to keep you interested for long if you don't have a lot of friends and/or family members. It's well-designed, though, and it gives you a good understanding of how to use the Wii Remote. And, being a game that comes free with a system, can you really complain?