The main reason people bought Wii Play is because it came with a free Wii remote. You don't usually see people handing over $20 or more for the game, because it's not worth that amount of money. It only lasts for a couple of minutes.
Wii Play consists of 9 mini games, but you have to unlock 8 of them when you start out. All you have to do to unlock the rest is to complete the earlier games. Since there are only 9 games, I'll go over each one in detail. Shooting range is a basic but strangely addictive target game. You shoot various types of targets for points. These targets include bulls eyes, discs, cans, and even UFOs trying to abduct Mii characters. While it's quite short, it's still one of the better games. Find mii plays a lot like 'Where's Wally?' but it's a lot harder. You're told to find a certain Mii character in a given time. Once you find them, time gets added on to the overall timer. The game can get boring but it's fun for a while.
Table tennis is a bit of a let down. It's actually a rally contest, in which you see how long you can return the ball to your opponent for. Not much to see here. Pose a mii is a very, very dull game. You're goal here is to match the shape in a bubble and fit into it. This is by far the worst game here, as well as the worst idea for a game in a long time. Laser hockey is basically pong meets hockey meets an 80's nightclub. All it is really is an air hockey game with bright neon lights. Quite fun. Fishing is where you collect different types of fish to gain points. Sometimes catching certain fish will give you extra points, which gives the game some challenge.
Billiards is a great game. Just pot the balls that you need into the pockets. The controls feel great and makes it worth while. Charge is a game that involves you riding on a cow taking down scare crows in a farm. Weird, but surprisingly fun. Tanks is a tactical shooter type game where you guide a tank through a level, defeating enemies to complete a stage. It gets hard even at level 5, but it can be enjoyable. Even though most of the games are fun, they don't last all that long, making for very little replay value.
Wii Play, while fun, is a questionable purchase. I'd recommend buying it if it comes with a Wii remote, but if you can't find a copy bundled with one, then you should save your money for something else.