I bought this as I cycle to work. It fits my 25 litre back pack snugly and has 2 elasticated straps with press studs to secure it. The studs are not well attached to the elastic straps and one parted company with the stud after I used the cover once. The flashing triangle doesn't come with batteries and the wiring is very thin; it won't respond well to wear and tear. However, it is bright when in use and has three modes, fully on, flashing and seizure inducing rapid flashing. It is fiddly to switch on and off once in place as the switch is on the inside of the cover in a pouch. The cover has got dirty very quickly from road grime and doesn't clean that well as you have to be careful not to wet the light's electrics. The white part inside the triangle has cracked in a couple of places during normal use. The cover appears to keep the worst of the weather out but I use a waterproof liner bag in my pack anyway.
It does the job to keep me visible in dark and wet weather but is very overpriced for what it is and the quality of some of the materials leaves a lot to be desired. Buy a waterproof pack cover from an outdoors shop and stick reflective tape on it and a LED flashing light, it will work out cheaper.