The routinely fitted flappy plastic spoke reflectors are a bit naff and ok but not brilliant - I take them off and use other ways of being seen side on: tyres with a reflective sidewall strip &/or spoke straws add unobtrusive reflectivity that provides almost a wall of light when the wheel spins
*caveat: wheel reflectivity is most visible as an effective entiry side on, but say you're cycling across a junction to your left OR right turning and a vehicle comes into view round a bend etc, side reflectivity at that moment is a heck of a lot more beneficial to you than front/rear facing lights.
Marginal Gains and all that*
I disagree with Accy that front and rear reflectors are rubbish too - rear particularly as they are A) a legal requirement and B) a mix of free light when illuminated by another and an absolute last resort means of being visible if you had no other lights working - possible with the rear that you may not notice a power failure ono mid ride. (Running more than one light is best tho, partly for this reason too)
I'm also an advocate of pedal reflectors;, even putting clip on platforms on one side of my roadie SPD pedals. Pedal reflectors and their uppy downy motion are instantly recognisable to all other road users as being uniquely cyclist in the way lights, hi viz vests, even red/white reflectors aren't necessarily so.
They give you another little edge in the battle to make the rest of the world see you and recognise you as a bike, likely to be on the road that they need to avoid.