I installed it despite my instincts to wait for beta 2. And oh if this is the direction Firefox is going, it may finally kick me to Chrome. And oh my, it crawls, it hangs, it uses 100,000K of memory as soon as you open it (although the old one uses about 80,000K, but it was fast so I didn't care. I've got 8GB of memory, so really I don't care, just make it fast) It is slightly but perceptibly slower switching from tab to tab. When I install an add-on, it doesn't switch to the add-ons tab or prompt me to restart. All of which could be simply add-on incompatibilities, but I haven't tested yet.
The only positive sign is that the add-ons page seems to be a work in progress, that seems to be causing some problems. I can only see 5 add-ons at a time, though, maybe it's my settings.
Also as someone noted on Lifehacker, they got "tabs on top" wrong and it doesn't give as much web page real estate as Chrome gives you.
BUT.. this is a beta after all, but I remember the 3.5 betas being really good.