I replaced my Dell optical mouse back in February with a Genius Wireless Traveller 600. It generally behaved well over these 6 months or so, but even early on it had started giving signs of poor quality. Within the first 2 or 3 months the scroll wheel started to malfunction. Since it's not really critical for me to use it I'd left it alone. Now, after about 6 months, the mouse pointer no longer functions properly. Regardless of my wishes, the pointer would move and jitter all by itself across the screen. It's so bad I had to reinstate my ol' trusty Microsoft IntelliMouse Explorer 3.0 which was for my old Pentium III machine.
It's funny how the decade old IntelliMouse is still working fine while the 6 month old Traveller has reached its last legs. Admittedly, the IntelliMouse isn't in top condition. It's also not as sensitive as the mice of today. But it gets the job done. Plus, it only acts up occasionally and that's after more than 4 years of good service.