I think the internet is a wonderful research tool and it is very useful for students of all ages. Younger students may not have a ride to the library or access to many book sources but a majority will have a computer in their home and it's probably easier and more fun for them to research topics online. However, it is very important for students to learn at an early age the dangers of trusting everything they read online. They need to know how to evaluate web sources and know how to judge whether a website is presenting reliable information or not. Internet research loses it's usefulness when students aren't looking in the right places for information.
Students in a social studies setting could use the internet to go on virtual tours or virtual field trips to give them a more hands on, creative approach to the lesson. It also can help put things in perspective. Science classes can do virtual labs online if actual labs aren't plausible (for cost purposes, or if they're simply too dangerous).
If I were teaching high school I'd be very afraid of students abusing technology in the classroom, messing around on the internet when they aren't supposed to, not paying attention to the lesson because they are too tempted by games or facebook. Also, in my high school there was a big legal issue because a student put offensive/blatantly incorrect information on a wikipedia page using a school computer and it was a nightmare for the teacher. I'll always be worried about something like that.