Ok, here's my oppinion on that.
The chance that your device will be completely bricked tends to zero.
There is a chance, don't forget that, but it's so low that you have a bigger chance of dropping your phone than that happening.
That build is first made by MS developers, then tested by them, then sent out to internal testers and tested by them, then goes out to even more testers
and so on and on, and then you get it in the fast ring. If they haven't found that bug, it's eather ultra-rare or non-existant.
What's a little more realistic to happen is getting an error during the upgrade and your phone becoming temporarily unusable - but you're a flash away from a working device in that case.
All in all, yes, you can brick your device completely, yes you can brick it temporarily, but the chances are low and I wouldn't worry about that.
As for me, I'll probably wait a few hours before I install it. Check the forums to see if people are having problems. If it's nothing special, I may not even install it.