Disadvantages
1. You're always going to be thinking about battery life.

2. No matter what anyone tells you, an LCD display does not look as good as a CRT display. The colors become smudged on a laptop screen.

3. It's damn-near impossible to upgrade the thing yourself. You’re going to have to send it in whenever you want it upgraded (RAM, new HD, etc.).

4. No matter what anyone tells, a laptop is always behind desktops when it comes to performance. You'll have to wait a few months or more before you can play games like Half Life 2, while desktop users have already beaten the game.

5. They're kind of expensive (if you want a really good one).

6. The keyboard and pointer are annoying as hell.

Advantages
1. You can take them virtually anywhere.

2. If you get a satellite internet connection, access to the internet becomes available in the Sahara Desert if you want.

3. They look a lot cooler than desktops (unless your desktop happens to be an Alienware)

4. When it comes down to word processing, the internet, and other low-memory tasks, a laptop is indistinguishable when compared to a desktop.

As you can see, I favor desktops greatly, but it all depends on what you need your machine for.