When it comes to hardware aesthetics, Sony has some of the best industrial design around. The 6.4-inch Xperia Z Ultra is undeniably attractive in its glass and metal casing. The software isn't so appealing, and not just because it's Android 4.2 - Sony's custom interface leaves a lot to be desired. If you'd like to run something closer to AOSP, not to mention a latter version of Android, the indefatigable CyanogenMod team is now offering nightly builds of CM 10.2.
Sony usually leaves their hardware alone in terms of bootloaders and other anti-modding methods, so current versions of the Xperia Z Ultra should be good to go if you've got a custom recovery installed. At the time of writing there's only one build available under the "togari" codename, the very first nightly for CyanogenMod 10.2 (Android 4.3). That should increase soon enough, with more stable builds becoming available sooner or later. As usual, nightly builds aren't for the faint of heart, and a backup of some kind is strongly recommended.
If you're ready to take the plunge, the single download is available from Get.CM. As always, you'll also need the applicable version of the flashable Gapps package if you want access to the Play Store and other Google services.