I probably wouldn't have paid for the shore excursion if it hadn't been included gratis, as it's very possible to get to Oia on your own by bus from Fira (or if you have a longer stay, via an apparently-stunning 3-hour hike). But, since the excursion was included, it certainly was convenient, as that bus only runs a couple times an hour and the cable car just to get up to Fira is absolutely mobbed when cruise passengers swarm it and it frequently has a deliriously long line. It can often be faster to instead take the long hike up the donkey trail to Fira by foot.