I passed the exam and am awaiting the endorsement
I've passed both the CISM and CRISC, which it turns out is ideal preparation for CISSP ISSMP 
Study Plan
Really just a follow on from CRISC and reread of CISSP stuff but I recommend:
- Reread of relevant CISSP areas - limited value  
- Utilized the ISACA CISM QA&E (Questions Answers & Explanations) - V. Good prep 
- Utilized the ISACA CRISC QA&E - V.V Good prep 
- Downloaded the ISC2 Exam Outline for the ISSMP, searched for, and read, references you are not clear on - limited value 
Test Question Preparation
The ISACA CISM/CRISC QA&E are essential, and very similar to the test in format - also a good number of questions - I personally think the ISACA QA&E are very expensive but also the best I have seen, if someone else is paying its the way to go!
Taking the Test
I think you have 3hrs but it took me just over half that time to get through the 125 q.
Only a few of the questions are really difficult, you can't go back so commit and move on!
Is it worth it? -  maybe
+ve
- Nice addition to my CISSP and CCSP and makes me look more a PM than a network bod I think 
- Helps you stand out amongst CISSPs who I imagine think its much harder than even CISSP (well that was my thinking anyway) 
- I find most people respect qualifications that don't have/understand - not saying its a good thing, just a thing 
Happy to answer questions!