Trackballs remain a niche product, partly due to the different set of mousing skills they supply, partly due to the price premium that most trackballs command and partly due to the lack of feature parity with even the cheapest mice.
Integrated scroll wheel works really well; Includes wrist rest; Ball can be juggled around for inspiration
Wrist rest may over-extend fingers; Scroll wheel feels a little loose; Still tethered by a cable