Winamp 5.5 is the biggest update to Winamp since, well, Winamp 5. Funny that.
There's a new skin, Bento, which puts the whole player into a single window. That's quite nice, because it's how I use the modern skin anyway. But the colour themes for it suck (to put it lightly), and the skin as a whole doesn't have the polished look of Winamp Modern. Perhaps it'll get better before release, which is scheduled for October 10 to celebrate the tenth anniversary of Winamp.
The only other major addition is support for album art. Unfortunately, the current default is to just dump the images in the same folder as your MP3s, with the name of the album as the name of the image. That's really unhelpful - especially since one of the reasons people use Winamp is because they like to organise their music their way, as opposed to the way iTunes or Windows Media Player want you to organise it. So, I've been going through and adding the cover images to the ID3v2 tags, because Winamp will at least read those covers, even if it doesn't write them.