I have always used and liked Logitech products, especially their keyboards and trackballs are awesome! I tried Kensington and even M$ trackballs at one point but none of them even come close to the reliability and quality of marble trackballs Logitech produces.

I never used a Saitek but a few friends from my former gaming clan highly recommended Saitek's keyboards and Joysticks so I believe they are pretty good as well.

I also have a wireless Logitech trackball, really cool device but like you said I got tired of odd movement, etc. when the battery was getting weak...all vendors claim the single AA battery should last up to 4 months and those using 2 AA batteries should last up to 6 months...yeah right, if you use your PC like 3-5hours week then may be but for bunch of geeks with PC addiction (like us), couple of months is a more realistic estimate.