How many people use these ?

There are always several floating around on the market and i've looked at them many times wondering ...... are they worth buying or do you always end up cursing that you've not brought the 'whatever's' along with you ?

I like the idea of them - to have hands and arms free, and just have a few notes, very small bit of change and keys on you - but ...... am i fooling myself, do i really need a bag with me instead, i'd love to hear your experiences on these little things.