I have an iPad and am actually on it right now! I use it to go online, play games with all the cool apps, I also use it as an ereader. I have my photos on it, my music, movies. I hardly ever go on my desktop computer anymore. I absolutely love it! I also have an itouch but just use it in my car. My kids are only 7 & 12 but both love to go on the iPad to play games they download. They have Nanos and love them but DD12 wants the new itouch for Xmas. She loves that it has a video camera on it.
For a teen I would say get the new itouch that just came out. It is great with lots of neat options and easy to carry around.