I use the Velious UI with horizontal hot bars and some custom windows for tracking and targeting.
My advice: pick a skin you like, tweak it and make it your own and use it. Here's what mine looks like:
http://i.imgur.com/G3199.jpg
My favorite part besides the horizontal hotbars I've grown overly accustomed to is the tracking window. Using the default one I can only see 4 mobs on my window at a time. It was terrible.