There are huge time savings to be had. With free IDE's there tends to be a lot more bugginess in general, gaps and things to work through.
As a Salesforce developer and consultant saving a couple of hours a month because the IDE is solid equals additional billable hours and reduction in frustration which is far in excess of the monthly fee.
There is a learning curve involved but there are a lot of videos that help with the features and its worth it when you get to the promised land.