I'd never considered it worth the cost, until recently I saw a wireless keyboard & mouse set (Silver Crest) for £13 in Lidl. Programmable extra keys on both - works well and excellent value.
Occassionally my trackball loses contact, but it is usually down to low battery. I find the convenience of sitting back with the trackball (and keyboard) on my knee, instead of having to be sat up at the desk, well worth the rare button press/battery change. And no lead to tangle me, or knock other things off the desk.