With Pass Plus, you need to take it within 1 year of passing your test and it then sticks with you for the rest of your driving career. It's what £100 with local council discount (if the Pass Plus instructor is registered)?
Gives you up to 30% discount with certain insurers which I'm yet to find but I've easily recouped the money I payed to do it over my 2 years of driving (having owned 2 RSs so been raped with insurance anyway). I'm 21 this year though, can't wait!