I have been running Ubuntu 12.04 with my / as btrfs for a couple of months now, with no trouble (and not even thinking about it truth be told). That said, I left my /home ext4 because I didn't know how much to trust it.
Did the same, btrfs for everything but home and will switch to ext4 with 12.10 because it feels much slower then the ext3 i had earlier.
But i'll be sure when i upgrade.
To be honest, what do i need btrfs for? I don't (and likely wouldn't) use all of its advanced features anyway..
In all the benchmarks ext4 seems to be a reasonable choice in terms of speed and stability.
I am looking forward to when we have fire and forget snapshotting system so that we all have rolling backups. I would also like the send snapshot features they have added recently for keeping multiple drives in sync (I currently use unison, but I think this could be faster/better).
Plus copy on write should be pretty cool (cp becomes as fast as mv), though I could see it allowing me to engage in some suboptimal behaviors of copying tons of stuff and counting on CoW to avoid actual duplication.
