I use XFS everywhere I can: out of the box, it requires no tuning to get near maximum performance, yet I generally have to tune ext3/4 to get what I want (and, largely, I still consider that a bit of voodoo).

Problem is, I run a dedicated server hosting company, and the majority of my customers either want CentOS 6.x or Debian Stable. Neither can install XFS as the root filesystem (but can use it for other filesystems during install, strangely enough; separate ext2/3/4 /boot doesn't fix the issue).

At home I have a mini-server that is 2x M550 128GB SSD, 2x es.2 2TB HDD, with the SSDs partitioned as 16GB md raid1 XFS for /, 256MB for ZFS ZIL, rest for ZFS L2ARC, and the 2x2TB as ZFS mirror; /tank and /home are on ZFS, and / is pretty much empty.

The only thing that would improve XFS at this point is if it supported optional checksumming and LZ4 compression on root filesystems, otherwise its basically perfect.

By the way, said mini-server? Dual core Haswell 3.x GHz, 16GB of DDR3-1600, from pressing the power button, going through BIOS, hammering enter to get past grub menu as fast as possible, it takes about 7 seconds to get to the login prompt; less than 3 of that is between leaving grub and getting to the prompt.