3D Spotlight have posted up a review of Microsofts Trackball Explorer, which is based on the technology behind their Intellimouse Explorer. Heres a bit:
The Explorer is one of the only trackballs out there to feature a traditional scroll wheel, a bonus for those who have grown up to love this feature of other Microsoft mice. It lies between the primary and secondary buttons, on the left of the wheel. Unfortunately it is still not as convenient as the Ring employed on the rival Kensington Turboring trackball. Having said that, the Thumb operated button is still comfortable to use, as well as functional. The same can be said, for the most part, for the primary button. On the far left of the device, this oversized button shares the same position as on most rival trackballs. Unfortunately the button is hinged at the back, and these results in it being somewhat awkward if you do not have your thumb right on the end of the button.