I work at a school district here in California that we've used OEM'd Gateway M285/M295 tablet PCs for a few years, and they've served their purpose. We've got hundreds of these units.
The good:
They do what they're advertised, and thats it. The math teachers like them, and thats about their only purpose.
The bad:
The drivers are funky. They seem to only work well with the factory image (LOADED with junkware). This makes creating and maintaining software images for the units cumbersome, not to mention the seemingly impossible task of finding a virgin version of WindowsXP Tablet Edition. The displays aren't very bright, which is painful in well-lit classrooms. The physical design (especially the keyboard) doesn't seem to hold well to a lot of use. Battery life is decent.
The worst:
The original OEM who we purchased them through has gone belly up. Even though our laptops had warranties, they all mean nothing now. Gateway doesn't support us. And the pens seem to fail at an alarming rate, and cost $79-99 each.
On the flipside, our district is moving to Mac, and thats been working very well. Apple's support for education makes you wonder why the rest of the industry hasn't caught up (its the customer, stupid!). We're at a standstill right now as to what to do with our aging tablets (that we get ZERO support for now). We don't want to get stuck in the same position as before, but Apple is also a proprietary system (but their support is awesome!!!).
I really, really like to see some good web-apps (ala Google Apps), or mutli-platform/open-source software that the students can use.