I've got a copy successfully up and running within a Virtual Machine (Virtual Server 2005 SP1), utilising 1GB RAM and 1 core of an AMD Athlon 4000+ Dual Core CPU. The install process took much less time than the old XP/Server 2003 installs and possibly even quicker than a Vista fresh install.
Once up and running it was very quick to get used to the improved GUI. Though I think there will be usability issues, especially with less able users - such that the taskbar items are now reduced to icons, with popup tooltips. But, Windows 7 is a public beta, with plenty of time to improve prior to an official release. Microsoft promises much better performance (compared to Vista) and has said that Windows 7 should run on lightweight hardware very well, for example Intel's Atom CPU.
Hopefully Windows 7 will take the best bits of Vista and mix them with new features and also what we're used to. One thing I'm already pleased about is that Task Manager now shows both the cached RAM and available RAM - whereas Vista only shows the former and XP (et al) displayed only the latter. Small changes like this save a few seconds of human processing power, as we no longer need to perform simple Maths calculations in our heads.
Personally, I'm looking forward to Windows 7's release, as it'll suit our 2 laptops better than Vista - if the performance improvements are rolled out. Hopefully Microsoft aren't 'dumbing' down Vista though, just to please those who complained about Vista's issues.
If you've grabbed all 2.5GB of the beta and have installed it - let me know what your first impressions are too.