these work really well, make sure you clean the surface (glass is best) with isopropyl alcohol to remove any residue/grease/oils which will make suction cups not work as well - especially inside a shower, otherwise they will simply fall off after a while
Have stuck one up for months and has not fallen (holds up a small caddy that holds a razor, wash scrubbie thing and a brush - I reckon less than 1kg total)
I have mine inside the shower on the shower glass, but well out of the way of direct water splashing/running on it. Steam hasn't affected it, not sure if excess water running onto it will.