It's not a complete computer replacement, so you would still need that, but it does take care of a lot of the tasks you can do on a laptop. For what I used my laptop for, it's a replacement. If you can find electronic copies of your textbooks, which a lot of publishers do offer (and cheaper, too), then even better.
As far as using it in class, it depends on what type of classes you're going to be taking. I'm an engineering major, and there's no way I'm going to try to type out equations/diagrams, so I find myself writing everything either way. If you can go your whole lecture by just typing, then a tablet would suffice.