I'm in the process of doing this, as my local council were giving grants so it only ended up costing me £25. Worth it just for the motorway experience imo.
Although when I rang up to get my insurance, they said it'd actually increase my premium. I'd had my license 3 months, but apparantly when you complete the pass plus it acts as a new license, meaning i'd have had it 0 months, pushing the price of the premium up by around £90 or so.
In the end I bought my insurance before i'd finished the pass plus course, so i'm not forking out any more, but i've still got the extra experience.