I like the new gui, but there are still some problems and inconsistencies. The major one for me is that you can't windowshade the playlist editor. I prefer to run winamp with the main window and playlist editor both windowshaded.
Aside from that, there's no published SDK yet (that I've found, anyway), and 2.x plugins aren't compatible. That means no MikroAMP, and no Lucidamp (<shameless-plug>my own plugin</shameless-plug>). Until Nullsoft releases an SDK, I'll be sticking to 2.x