In its favour: it counts laps accurately, and it's easy to use with just one hand. The habit of wearing it on the inside of the middle finger and pushing at the moment of turn is easy to grasp and has stuck with me even now the unit is dead.
Against: the digits are very faint and small (the whole unit is only about 3cm long) and not easy to see by pool light. The button doesn't give enoough tactile feedback in the heat of the moment and when I started I couldn't tell whether I was pushing on the display glass or the button. After a month the button came out (together with the switch operating pin which I hope has been sucked into the filtration rather than lying waiting for someone's foot). Then the display died, filled full of water and that was the end.
Overall - fun whilst it lasted, now I'm trying a Timex Ironman 150 (but the istructions actually say don't push the buttons underwater, so it might not survive! And it can't be done with one hand either).