Truly a great value and one of the most important purchases to make!
We bought the London Pass for the second time in August, 2018. London is incredibly busy this time of year and I would recommend visiting another time but if you do go it's absolutely required to have the London Pass. The lines are absolutely insane otherwise.
The amount of value you get for this pass is truly unsurpassed. You can see almost every attraction in the city and skip the long lines which is a huge issue in the summer months.
This time we had just a half day and it was still very useful. We used the London Pass to go on a Thames River Cruise, we saw the Shard, we went on the Hop-On Hop-Off bus tour, the Tower of London, the London Bridge Experience and several other things.
If you have a full day it's obviously even more worthwhile, but don't worry about the cost even if you're only here half a day like us. We still got great value in a half day and I would probably pay even more to avoid those lines!