I've always understood that you can't really measure wear on a cassette or chain rings, like you can a chain. The deciding factor seems to be "does the chain jump?". Visually, the cassette looks very used, with evidence of a lot of shifts on it (scratches and wear marks on the sides of each sprocket). The chain should definitely not skip under only moderate load (like into a wind) and, if you can rule-out things like ghost shifting from a mis-adjusted derailleur, you can probably put this cassette on the list for replacement.