It's pretty speedy and responsive on my 2.4ghz C2D ThinkPad X200. Being a Developer Preview, it's also pretty useless. But, it does work as in today's keynote presentations. You can install apps on it.
Not all the apps work, some are just mockups and don't have any functionality (though they seem to be legitimately pulling data from the network, according to Wireshark).
It's not ready for regular use by anyone's standards, for sure. But it's very promising, very speedy, and has a pretty low memory footprint.
I'm OK with the demo goof ups.