Pros: The Fish works decently as a trackball-based device. It was recognized immediately by Windows XP without having to install any specific drivers.
The trackball itself is controlled by the thumb and is well placed; the ball physically moves very easily (but see below under Cons).
The two buttons are on the opposite side and are easily clicked (again, see below).
The device fits my hand nicely and is the cable is long enough for my purposes.

Cons: The biggest issue is with the trackball: while the ball does move fluidly, the computer's cursor doesn't always respond to your movement. After some testing with holding the device at various angles, it appears that it works best when holding the Fish flat as if it were a regular trackball, or at a slightly downward angle. This is a real shame, because it means having a handheld device but having to treat it like a normal desktop variety.
The buttons are also too easily activated. (Another reviewer reported the same problem.) Since your grip of the device is a combination of the thumb, which controls the ball, and your fingers, 2 of which are held against buttons, this is a bad thing.
Finally, the lack of a scroll wheel makes this device cumbersome when reading through web pages or documents. I found that my hand tired out easily because I had to constantly hold down with my trigger finger while moving the cursor with my thumb. This changes your grip of the device and it became uncomfortable. One way to get around this is to go into your mouse properties and switch the "left" and "right" mouse buttons - this way the primary is your middle finger, which was easy for me to hold down on for a long time without affecting my control of the ball. Another, I guess, is to just position the cursor over the window's scroll buttons and click the up/down arrow instead of dragging the scrollbar.
Note that switching the L+R mouse buttons in the Control Panel will also cause your other mouse devices to switch!

Other Thoughts: I should note that I bought this device specifically for use with my laptop while I'm away from my docking station. I wanted a trackball device to avoid RSI.
My hope was that it would be easier to use than the laptop's built-in trackpoint and touchpad.
After playing with this for a while, I'd have to say it's not as easy to use as I had hoped, making it more difficult for me to justify bringing this device whenever I roam around.

Bottomline: It's definitely worth the price I paid ($7.99) and I do expect to continue using this, but this is not my ideal device.

Pros: The Fish is actually optical (you can see the red light in it), and it's resolution is fine.
Plug and swim..err..play (tested under XP)
Would be nice to use for RSI people as a change, but could also induce RSI.
Works fine if no flat surface is available.

Cons: The ball is hard, and has some mechanical play in it. Not handy to draw straight lines in graphics applications.
The two buttons (L+R mouseclick) are loose and too easy to operate, you have to take care not to click all over.
Can't swim.

Other Thoughts: I needed it to build into an application without flat surfaces, for this it is ideal.
The build quality is quite solide, mostly due to its shape. When unscrewed, the PCB looks proberly designed and built.

Bottomline: Although the box says: "The mouse is dead, long live the fish" it's not recommended as a complete mouse replacement, but otherwise, if you need something like it, get it, it works fine.