I don't have a tablet yet, but I can see clearly how I would use one:
- reading news/books on the train. I can do this with my android phone now. The comparison of phone to tablet for this is a bit of a wash. However, netbooks/notebooks don't suffice. You need to be sitting down, and the keyboard is a useless, awkward mass. On a crowded train, I can't even get a notebook out of my bag.
- watching videos/playing games on the train. I do this less often, but a tablet would be marginally better than a phone. Again, the notebook form-factor is useless and awkward.
- Showing videos to my classes on the in-room monitors/projectors. The tablet has more possibilities than does the phone (USB host, external memory, more video connectors). Again, the keyboard is unneeded.
- Light document editing at my desk -- using a USB keyboard I leave at the school. Here the notebook has an advantage in terms of software available, but if google docs suffices for lessons plans, then the tablet is just as good.
- Email from my desk. Same as notebook as long as I have the aforementioned USB keyboard.
- Web browsing from desk.
So, for me, the tablet does double duty -- PVR/notebook and transit eReader/PVR/game/email/web device.
The price, though. It seems to me that the touchscreen and the novelty are the big differences between netbooks and tablets (that, and the missing keyboard). So I'd expect a USD 100 or so premium for the tablet. Notion Ink's Adam is looking like the ideal device at the moment, though they need to demonstrate long-term viability and service and support (like offering an upgrade to honeycomb soon with the ability to forego their Eden UI entirely.)