I've been stuck on UF since I discovered it almost 2 years ago. Love it so much I actually paid for it twice! ;p
The greatest thing about UF for me is the programmable Transforms, Fractals, and Coloring Algorithms, not to mention custom-design gradients. I'm a math-head and I love exploring different types of fractal images via equations. I'm convinced that if you know what you're doing you can create virtually anything in UF!
Also have Apophysis 2.02, but the interface is a little intimidating in spots.