I have been using a Logitech Cordless TrackMan for around 5-6 years and have had very little trouble with it. It's travelled all around the world with me and has been subjected to all manner of 'travelling' abuse. It's had that much use that the silver paint on the buttons has worn away but it is always reliable.
I use it with a laptop as a mouse isn't always practical. Once you get used to using a trackball you never go back to a mouse! I have a different model for use on my Desktop at home too. The best thing about a trackball over a mouse is maintaining cursor position and accuracy during button pressing. You can position the cursor were you need it remove you finger from the ball, then click the required button. This is vital for photo or video editing software etc.
If I was being critical. (and I often am) My only complaint would be that the receiver is a bit bulky and the range isn't great. This one is ideal for desktops where portability of the receiver isn't an issue but for laptops, buy the Graphite. Logitech M570 Wireless Trackball - Graphite
I intend upgrading to the graphite for my laptop and using this model for my desktop.