Living in Exeter, a half an hour's walk from St David's (the main station), I can be in Okehampton in less time using the A30 even with the summer tourist traffic. If I drive to the station then parking is limited and expensive, added to whatever the train ticket will cost. So locally it will probably be more useful to commuter into Exeter
However, it does offer an excellent opportunity for a weekend walk across Dartmoor between Okehampton and Ivybridge main line rail station, wild camping or overnighting in Postbridge (pub, B&Bs and YHA).