We have a fairly complex model for shipping prices depending on: weight, spend and destination. Native Shopify couldn't cope so we tried Calcurates and it's wonderfully flexible and supported all our requirements. Configuration took a little time, with setup and testing each zone but the support team couldn't have been more helpful and responsive. I definitely recommend reaching out to the team who will walk you through the full capabilities of the tool.
I think the speed of the checkout process has increased — we have less abandoned carts for sure. I'd highly recommend Calcurates to anyone for whom native Shopify doesn't have sufficient functionality.