I'm on my third load of fully synth (castrol as it happens). Very happy, slicker gear changes for the first 2000 miles, after that it behaves much like semi-synth. I find the semi starts to behave like an older oil after 1000 miles.
Oh, and it doesn't burn off so quickly as the semi, so I don't check it nearly so much.
Worth the difference...? For a leisure rider I'd say get the cheapest and change more often as condensation is a bigger problem than oil quality. Nothing beats new oil.