It might be because the actual release is half a year away, in early summer, but the impression we got when playing with the HP TouchPad was that it was slow. Not incredibly slow, but compared to, say, the Motorola Xoom or an iPad, you'll notice slight delays between your touch and things actually moving around.
The actual UI elements and the way things move around are great, so long as you're a fan of the way webOS does things (I am). 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 instance is kinda slow. Again, it's not unusably slow, if you've used the Pre compared to a faster phone, say, a Nexus S or a Motorola Atrix or an iPhone 4, you'll know how the TouchPad compares to the Motorola Xoom. 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 would be awesome.
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.