Yes it's very much worth a visit - about £15 an adult according to my Brickie who went there on Sunday. Lots to see - the most impressive bit to me is the huge wooden shed where ships used to be built - but the rope walk must come a close second. Usually loads of things going on there as well.
Eons ago I used to occasionally work there when it was operational. The Royal Naval Stores were controlled by our computers, there, and also Rosyth, Portsmouth and Devonport.