Navigating round the NWZ-A815 is again very similar to the NW-A805, with directional buttons used to navigate the grid based menus. There are a few very minor cosmetic differences like larger icons and a different background, but essentially it's the same. With the advent of the iPod Touch and its incredible touch driven interface and the latest nano and its new CoverFlow and half list/half thumbnail menus, the NWZ-A815's menus do look a bit behind the times on the funkiness scale. However, for simply getting to where you want to be, they are still without fault.
To accompany the new drag and drop interface, a new option has appeared in the music library that allows you to browse by folder and file name. So, if you can't find your music using the tag generated library, you can at least go back to basics and pick it out by hand.
Music can also be added using the sync facility on Windows Media Player 11, a copy of which is provided on CD, which, even if you don't use it to organise your music, adds some useful extra functionality to Windows Explorer. Namely, when you try to copy music or video to the player it will pop-up a message telling you if the file isn't supported by the player. This is particularly useful for video, which tends to be encoded in all sorts of esoteric formats. Also, for all your open sorcerers out there, you'll be glad to hear the NWZ range also plays nice with Linux as well.
As for music formats, I'm sure you won't be surprised to hear that MP3 is now supported and is joined by WMA and AAC (DRM-less versions thereof, of course). Notable, one might even say lamentable, by its absence, however, is Sony's ATRAC format. Even though we don't like to see proprietary formats used exclusively, to drop them completely is almost as bad and, given the superior sound quality of ATRAC compared to MP3 and WMA, it's an even more troubling loss. At least Sony do bundle an ATRAC to MP3 conversion tool that will convert your entire library in one go, saving you that hassle.
Listening to the two players, there seems to be little discernible difference between the two, which really is a testament to how good the original NW-A805 was in the first place. That said, one thing I didn't notice the first time around was a hiss that is introduced when anything but the standard EQ settings are used on the NW-A805. Thankfully this is something that's been cleaned up on the NWZ-A815, so it remains hiss free regardless of how much you pump that Clear Bass setting.