There is a rumbling debate going on in the Windows Mobile world as to which Windows Mobile overlay is the best. Sony Ericsson’s Panels concept is certainly novel, but HTC’s TouchFlo3D shows the benefit of being a 3rd generation product.
It seems Xperia owners will be the first to answer the question, as they will be able to experience the software side by side. While the Xperia has not been officially announced yet quite a few pre-release devices are floating around, and the happy hackers at XDA-Developers have been making fast work of them. XDA member Nelshh has done some of the work of porting the interface to the WVGA screen. The process is however still pretty complicated and unfortunately wont bring the 3d accelerated goodness of TouchFlo3D, but only the more mundane 2D version.
If however you are rearing to answer the question of who has developed the best UI (and of course you have an Xperia handy), pop over to this XDA-Developers thread for the relevant files and instructions.