I solely use the Kensington SlimBlade on my desktop.

It's fantastic especially for general purpose work and browsing, browsing is so much more easier with the SlimBlade, turning the ball to scroll.

The only time I find a mouse more efficient is if doing lots of selecting text and copy/pasting or diagram type apps. I have a backup mouse for when a mouse is more useful but almost never use it.

I was getting minor RSI which is why I originally thought i'd try a trackball. Glad I got one, it's so much more comfortable and wrists / arms now stay relativity still. It's just a shame the form factor isn't that great for carrying about with a laptop.