I have lost so many rear bike lights, finally a bike light that stays clipped on my backpack straps - the metal clip makes all the difference compared to all the other lights with a plastic clip. Additionally its simply to use because it doesn't have too many settings so I can turn it on and off without taking off my pack. There were brighter ones of these available but I don't see the point this one is already really bright.
I'm super pleased with this light easy to move between bikes (and detach to prevent theft), bright, and versatile (mount to post, clip to pack, etc). And rechargeable means no more cost or hassle of replacing batteries - you don't have to sweat it when you inevitably leave it on after a ride. I've seen reviews mention that it can't be angled, but it's bright enough and has a broad enough visibility spread that I don't think it's a real issue.