Seb answered better than I can, but just to add from someone who has a 2x29 v10, mullet Bullit, + 2x275 Bronson (all santa cruz, though I'm not sponsored, hint hint SC ;)
I really like the mullet of all the options. I agree that steering redirecting with the 29 front feels more natural, i don't get thrown around on roots/rocks and have better traction to steer my bike.
The 27.5 rear just results in my ass getting buzzed less (i'm 5'10), more forgiveness when i screw up on jumps and rear comes up too much. Plush forks save me each time :D
Not a huge deal for me with the 29'er rear, i adjust my riding style fairly quickly, but given the choice, I'm currently pro mullet, albeit with limited set of bikes to compare with.
For me only downsides of mullet are that you can't move the front tire to the back to live out the end of its life. You can stretch out a 27.5 spare tube onto a 29 if need be, I hear. However as long as you run "proper" tires (DD) you shouldn't need tubes.