There are quite a lot of threads already on this - try doing a search.
But to try and quickly summerise what people seem to say:
*If you have your own insurance (ie: its in your name) then you can make quite big savings - depends on the insurance company though. Some do offer big discounts, others offer nothing - so shop around.
*If you are one some elses insurance as a named driver, you won't save any money doing it, no insurance companies do any discount for a named driver with pass plus.
*Some councils pay for some of the pass plus fee to try and get more people to do it - have a look to see if your council do.
*Also, you need to decide if it worth the extra driving 'experience' you will gain, even if there are no monetary benefits. A lot of people say its good experience. Particulary if you havn't have much experience of different conditions (bad weather, night driving etc).
Personally, I didn't do it, as I was going on my mums insurance, so wouldn't have any discount. Had done all the things covered on pass plus already except motorway driving so just went on the motorway with my dad.