I use the Evolution trackball from Itac Systems (Mouse-Trak). It's not cheap, but extremely rugged, and has 6 easy-to-use buttons. Three are the normal ones, one is for fine/coarse motion (which I never use myself), and the other two give you "drag lock" and "double click". I like the drag lock for exactly the reason you state, of not having to hold down a button while dragging an object. The double click is convenient, but often I forget to use it.
But here's my biggest reason for liking the track ball with Ironcad. When right-clicking for a context menu or for the Triball, with a mouse it's very easy to produce an unintended cursor movement at the moment you click the mouse button. With a trackball, you can simply remove your finger(s) from the ball before clicking a button, thus ensuring absolutely no unintended movement. Works great for me.
I had used a trackball before IC, but had gone back to a mouse (as my first TB was Kensington with only two buttons). After using IC a few months, I decided to get the Evolution and haven't looked back. BTW, it hasn't at all spoiled my ease and familiarity with a standard mouse, which is what I have on my home machine. [Now, if I can just get the hang of that darn touchpad on my laptop...]
Regards,
Peter
