Very Good Product,
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This review is from: Knog Frog Strobe 1 LED Rear Light (Sports)
Very bright yet very discreet when switched off. Rubberised clip straps make it easy to attach to seatpost. At least three modes: one constant and two flashing. I think battery life is OK; also you can get a replacement pack of 12 CR2032's for less than 3 quid including postage.
The switch activates easily for example by resting your bike against a pole, but once you're aware of this you can take sensible measures to avoid, like a simple twist in the other direction when not in use.
I use this as my spare in case the main light fails - always sensible to have two rear lights on at the same time at night in London traffic anyway, I think.