Unless you want to reshape your array. Then md-raid is superior. Btrfs's raid implementation has had some pretty bad issues recently and has been declared "experimental" as a result.
Apologies, I only saw the recent mailing list post which basically said "we're removing the ability to even compile support for raid56". I didn't mean to say that btrfs was overly-zealous in their trust of new code (I personally am super excited for the future of btrfs).
Other raid levels (raid0, raid1, raid10) work fine and are considered stable -- at least as stable as the filesystem itself.
Either way, if you care about the data, you must backup, no matter what fs you use.