Knew this new format (LZMA-based) via Slackware, Slackware will be starting using XZ to package. I tried to emerge (install) it on Gentoo, but it's masked and blocking with lzma-utils. So, I downloaded and compiled it. The version I used is 4.999.8beta.

Here is a quick result, I used Linux kernel source 2.6.29.3:
              gzip  bzip2     xz
compression 22.1s 78.1s 322.9s
72.6M 56.5M 48.2M
decomprssion 4.0s 19.3s 7.4s

Original tar size is 336MB.

XZ compression time is about 4 times slower than bzip2 (1.0.5), 3 times faster than bzip2 in decompression, twice slower than gzip(1.3.12) in decompression. If consider as a file distributer, the numbers are great. We can ignore the compression time because compression only does once, but transmission and decompression do once per request. It save lots of bandwidth and time. Though it's almost twice slower than gzip, but not many connections can transfer 24.4MB in 3.4 seconds.