I am using the Ubuntu beta since the early Alpha versions and I should admit that everything is going into place very well. Actually using it, you'd never say it's a beta given how polished and smooth the user experience is. A little bloating on the other hand is pervading the desktop setup and maybe too many services are active by default. With 512mb RAM you will need to disable something to have a better experience but compared to some competing OS the situation is really good.
Comparing the Ubuntu 8.04 beta to my other Debian Lenny box with comparable hardware (laptops with 1gb ram and centrino 1.8GHz) Ubuntu feels much faster doing everything. I don't know if it's the kernel 2.6.24 with CFS or some optimization of the libc or something else but the difference is night and day. Debian should care a bit more about performance if it wants to stay the UNIVERSAL OS...
So, up to now: really good and two thumbs up for this "beta" (just a bit different from the stability of KDE 4 beta!....)
Happy dowloading!
marco