Although the Touchpad I played with is not the final version, my impression of the HP TouchPad is that it is slow. Not incredibly slow. But compared to, say, the Motorola Xoom or an iPad, I noticed slight delays between my touch and the interface moving around.
The interface looks fantastic. And so are the animations. The screen fonts look natural and "Pre-ish" on the 1024x768 screen, even if they're not super sharp. Opening, moving around and dismissing apps functions just the way it does on the Pre now, and is intuitive enough to make it onto tablets without any sort of problems. The only major issue is the speed.
This might be because of the fact that the TouchPad isn't done, and has six months of development time left. But every app, every notification and every multitasking feels that way. This is surprising since the TouchPad has a 1.2GHz dual core processor, but, webOS has never been a particularly speedy OS.
The HP folks wouldn't let anyone hold the tablet so we can't tell how natural it feels in your hand, weight-wise and size-wise, but gesturing around the screen was pretty standard. Actually seeing the Pre 3 sync web pages through their Bluetooth connection to the TouchPad is cool, as is the ability to answer phone calls and respond to texts using the tablet. More interactivity between the two devices seems like a natural way for the two to work together.
The end result of the TouchPad is that it definitely does take webOS onto tablets-which is what people've wanted-and added various cool features like the ability to talk to the Pre wirelessly. It doesn't, however, add a bunch of really new UI elements or features that makes the TouchPad a must have when compared to Google or Apple's upcoming tablets. And since the iPad 2 and better Android tablets will be on the streets come this summer, no one knows how the WebOS powered Touchpad will compete.
Update: After actually holding the TouchPad, I can say it's deceptively heavy. It's around the same weight as the iPad, but you'd think it would be lighter because the back is made of plastic. That said, it's not overly heavy, just heavier than you'd think by looking.
As far as the UI goes, it really is a good comparison with this SAT analogy. HP TouchPad : Motorola Xoom :: HP Pre : Any Android Phone. Both have taken their core UI principles and blown them up to tablet size. The Xoom, from our hands on, is a big Android device with better multitasking and more stuff visible at once. The TouchPad is a big Pre with more processing power and more stuff visible at once.