I cannot speak for the left-handed version, but the right-handed Felcos are ergonomically brilliant. The angle of the cutting head helps reduce the risk of muscular-skeletal injuries and maximises comfort. The cushion and shock absorber soften the impact as you finish the cut, protecting the hand and wrist, and these tools are lightweight.
As with all Felco secateurs, these models have a groove to remove sap and debris after each cut, preventing the blades sticking when cutting "sappy" wood. There is also a wire notch that allows small wires to be cut without damaging the cutting edge.
The difference between Felco 9 and Felco 10 is the revolving handle on the latter. This should spread muscular effort across all fingers and Felco reckons it reduces overall cutting effort by 30 per cent, lowering the risk of tendonitis and carpal tunnel syndrome.
For me, the best bit is the ease of sharpening. Felco offers sharpening tools and stones as well as a maintenance product and grease. In the event of a part being damaged, all bits are replaceable - from handles and blades to cushions, shock absorbers, springs and thumb catch.