I walk home every night watching cyclists on their commute covered with these little flashy things as suggested above and IMO almost all are very impressive when you look at them square on and all are completely useless in the real world when you are looking at an angle. Also if you hang it off your bag and it points the wrong way or if they are mounted behind a mud guard any light is useless - sounds obvious but sooo many people do this kind of thing.

Dont get some silly super bright tiny little flashing thing, they are for kids to look cool, the lit up area is too small and they just get lost in the sea of other lights. Who cares if it leaves spots on your eyes when you look at it, thats not the aim. Get one with a large lit up area, that stays on constantly and has a good refractor to make it visible when not viewed square on. Take care to mount it at eye level for a driver and fix it so it is level with the road and not obscured by your legs or anything else when viewed from behind AND the side.

If you want to have a flasher too then its a good idea becasue it identifies you as a bike but dont rely on something with a lit up area smaller than your small finger nail and is only on half the time.