A laptop is just inherently so much more awesome than a desktop. The portability factor is just... sweet. You can take your computing anywhere.

The only disadvantages I would even consider is limited upgradability (a word?) and somewhat limited functionality (a corollary to the former). You find 2 USB ports are often not enough (depending on how much you use it, that is). You probably can't do advanced video and audio editing on it either (at least, I can't as well as I'd like to) and you probably can't play your PS2 off of it (once again, at least I can't).

Advantages are numerous. Portability, portability, portability. You can take it anywhere. Especially those 12" Powerbooks and the 10" Toshiba Notepads. I'd recommend the Notepad if you're into writing notes by hand and stuff - you just write on the screen, make little sketches and stuff... it's awesome, ideal even for a physics or engineering student who's gotta make a lot of diagrams (like me, except I don't have one. Grr...) Battery life isn't too often an issue - you're not really going to need more than 3 hours unless you're on a really long plane trip and you're really bored... however, if you're concerned about battery life, look into Intel's Centrino chip - it's supposed to reduce battery strain and stuff.

Best brands in my opinion are HP and Toshiba. Dell has these weird widescreen laptops that have messed up video cards that work with basically no applications. Sony is just way too expensive, and not as compatible with some external hardware. Alienware is great, but you usually don't need the souped up features unless you're a hardcore gamer or post-production specialist. My HP has served me quite well, and I got it for a neat $1300...

Meh, that's all. Do your own research as well.