I've got a Fujitsu that I use on a daily basis (though for general "office" use - I almost never use paper anymore - and for software testing, not for development) and regularly interact with a client who has some Dell and Motion tablets. My Fujitsu has been rock solid; it's got a Core2 Duo 2000-something, 2 GB of RAM, etc. My biggest issues with it from a development perspective are lack of screen real estate (1024x768) and very small keyboard - though that's a common complaint on just about any laptop. You can get a docking station, but then why have a tablet in the first place? :) I purchased a modular bay battery since I rarely use the DVD drive - if I can't download it or transfer via a thumb drive/SD card, I don't bother. With the extra battery, I get about 6 hours of battery life vs. 2 on the standard battery alone.
The new Dells are thinner and lighter; power-wise they're probably about the same. They also have the benefit of a dual-mode screen - it is touch sensitive in addition to the tablet pen. For my client, this is a good thing for what they're doing, and they get it out of the box.
Motion are now defunct (bought by Dell) and their big thing was that they were super-light and super-thin BUT didn't have a keyboard. Again, this was perfect for the niche goals for the client, but made daily use somewhat problematic.