I would highly recommend using them for "hard" bookings, like Cinderella's Castle Character meals, Illuminations cruises, etc. They will tell you that they can't guarantee that they will get the booking for you, but that they have good luck. We used them in the past and have used them this year and have snagged a few hard to get things so far. If you are leery about missing out on something, you can have them try for those bookings while you try on your own as well, knowing that you can cancel one later.
As to other PS, it's up to you. We gave them a list of what we were interested in, and they went to town. It was nice in that we didn't have to worry about anything with it. I guess I would look at it this way - it's part of the service you are paying extra for!
Good luck and have a great time in the planning.