been using kensington's trackballs for over 5 years now, their customer service is next to none in US. Any problem with the trackball they send you a new one (5 year warranty).
I've used the expert trackball but purchased the slimblade trackball as soon as it came out, it had bunch of problems at first but now they have been around for a few years already and all problems have been ironed out. I love it.
