This is the same as asking it Television is a good thing. It really depends on who you are and what you are expecting to get from it. TV can be seen as a bod thing if you become too dependant on it for your daily life and your daily learning. The TV has been viewed as a "goggle box" that leads to the destruction of the viewers mind, it has also been seen as an educational tool. If we think about TV a little bit we can see that watching so called "pulp" TV shows (with little educational value) too often (or always) could be a bad thing, but then if we think about TV as being educational we can see that most schools and teachers show it to kids to educate them (ie: National Geographic, PBS etc.)
The same goes for the Internet (pls note: capital I not lowercase because in this case it is a proper noun).
So the answer comes down to what you watch on the net and not the net itself. It's a bit like the gun debate: (ie: guns don't kill people, people kill people). So regards the Internet, it's not the net that is the problem, it is the content on the net that can be the problem.