I have used so many mice over the years and gone through a great deal of trying to find the most reliable and comfortable one for me.
Here is my summary:
Intellimouse ball mouse - very good in its day in both comfort and accuracy.
Intellimouse 2.0 Explorer Opitcal - when it first came out it felt like sheer perfection.
Razer Boomslang - Extremely precise and fast but not very comfortable.
Intellimouse 1.1 Explorer Optical - much like the 2.0 but not as comfortable for me.
Intellimouse 3.0 Explorer Optical - Again, felt like sheer perfection and had big improvements over the 2.0
Logitech MX500 - Felt like a big leap in accuracy over the 3.0 Explorer but I had issues with the left mouse button sticking.
Razer Diamondback - Extremely fast and accurate but not very comfortable.
Logitech MX518 - Like the MX500 but with better DPI but again had problems with left button sticking and becoming faulty.
Razer Copperhead - Comfortable, precise and very reliable.
Intellmouse 3.0 Explorer Optical NEW version - Horrible casing material (rough instead of the old smooth plastic), loose side buttons which easily broke and more problems with the mouse scroll.
I have gone back to a new Copperhead because for me it has been the most reliable all round mouse. I plan on trying something else again though when it dies.