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.
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.
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.