As wrote about earlier this month, Ubuntu developers planned to bring up an XZ discussion at UDS. The discussion happened today in Copenhagen, but there was no definitive conclusion about moving from Gzip to XZ compression for their Debian packages in the next release.
XZ offers a better compression ratio than Gzip and some other compression alternatives, which would mean the DEB packages are smaller in size. This smaller size would benefit end-users in having a shorter download time and burn through less bandwidth in obtaining new/updated packages. Ubuntu mirrors would also save space, albeit this isn't one of their principal reasons for wanting to switch to XZ compression.
Concerned raised about using XZ packages is that Debian hasn't yet switched their packaging to XZ (though they eventually plan to) and potentially greater CPU and memory usage with extracting XZ-compressed packages. The primary concern expressed is that if it takes significantly longer to decompress XZ packages on some hardware, this may hold back their adoption of the new compression scheme. If it takes measurably more time in the end to decompress and install the packages while factoring in a shorter download time, their XZ plans will be aborted for Ubuntu 13.04.