I haven't seen the other brands in the flesh, I have a pandora but mum told me she saw another kind and didn't look as good a quality, I guess thats why the pandora charms are higher cost.
I have bought a charm off e-bay, they all can fit on each bracelet so you can mix and match but I have found you can tell the e-bay charm as it just doesn't seem as good as the authentic pandora charms.