A frequent question on this forum, "Is the London Pass worth it?", can ever only be answered by comparing admissions cost of where you want to go with the cost of a Pass. In general, the longer the time period of your pass, the lower cost/day and the higher chance of receiving good value.
I'm headed for Norway cruise and then 6 days in London with friends in short while. We've firmed up our itinerary. I already knew from experience that a 6 day pass would provide value for me. I went ahead and priced out the 10 locations covered by the pass that we know we are going to for comparison. Admissions cost for those 10 locations is £272.60. The cost for the 6 day London pass was £144.
One of the biggest complaints about the London Pass is that people try to put too much in one day in order to "get their money's worth." When you are using it over a longer period of time that stress is eliminated. In fact, we have plenty of flexible time in our schedule and will probably do more than the 10 locations/activities that we priced.
Another side benefit is that it can allow a person the opportunity to try something that they wouldn't pay additional money for. In my case, it is the London Eye. I've been to London multiple times and never felt that the cost of the London Eye would be worth it to me. It is now covered under the London Pass, so I am going to give it a try.
Bottom Line: There is no one answer about the London Pass. Each visitor needs to price things out and see if the value is there for him/her. The longer passes are priced at a better cost/day ratio than the shorter ones.