Last week I had a 1-day "test drive" using my colleague's trackball instead of a conventional mouse device. Today this experiment ended up in buying an indentical trackball (Logitech TrackMan Wheel) for daily office use.
This device has a "transitional" mouse-to-trackball design with mouse-like left and right buttons and a scroll wheel in the center, which I eventually found very handy. However, at first the new device confused me resembling a ball mouse inverted inside-out, with a ball directly under my thumb. In a couple of hours though, the trackball felt as natural as my old optical mouse.
I spent the rest of the test-drive day enjoying high cursor speed coupled with high-precision cursor positioning and pleasant tactual sensations. In terms of my everyday tasks, it became much easier to quickly move the cursor across windows on my 2 screens. At the same time, I gained more control in selecting text and graphics. And what's important, my usual strain in the right arm and shoulder were gone because I didn't need to move around the entire device any longer.
Logitech also offers a similar cordless trackball. Perhaps, some users would find it critical to have no "tail" connected to a trackball. For me a cord is not an issue because, as I already said, I do not move a trackball around and it lies still on my desk.
The round-up is that I would recommend trying a trackball for typical techwriting jobs. So far I found no down-sides of using a trackball as compared to a mouse (I'll make an update if I find any flaws).
UPD 1: This (and the similar cordless model) are "pure" trackballs meaning they do not have mouse-like sensors on the bottom. These trackballs lie still on the desk, giving you a fine opportunity to click without the risk of accidentally moving your cursor away.
UPD 2: I've been using my new trackball for 2 weeks now, and I'm still happy with it. After some fine-tuning of speed / acceleration settings the device feels just fine.
UPD 3: If I ever decide to move on with my "freaky mouse" experiments, I'll definitely try a foot mouse!