As far as rear lights go this is the daddy of all the self- contained units. With 10 bright LEDs powered from just two AA batteries, the run time in excess of 100 hours (we actually got 102 hours) in legal flashing mode is simply amazing.
The supplied mountings will fit seatposts from 24mm (Cateye claim 24.5mm) up to 32.5mm, with an optional mount available to fit up to 34.5mm. There's also a clip mounting inside the box to enable you to fix the light to a bag or jacket.
Two buttons on the side of the light control the two individual strips of five LEDs, which are laid out in two rows with three pointing rearwards, and a single LED at either end pointing at 90 degrees in each row. The modes are flashing, constant, sequential and fast sequential (in that order), making 16 ways to have your light turned on.
The single best thing about this light is that it's bright, and bright at almost all viewing angles. Add to that the fact that it's well sealed, very well made, runs for ages and has a good mounting system, and this is a clear winner.