Are you two adults and two children? How old are the children? They may not be interested in running from covered sight to covered sight, which you might have to do to make the pass pay off. Time spent walking to the nearest Underground station, riding the Tube and then walking to your next target (London is really large) is time not available for actually visiting places.
Keep in mind that London has a long list of large, top-flight museums that are free/donation requested. The pass will not help with those, and the time you spend at places like the British Museum will be time not available to go to places covered by the pass.
If you're attracted by inclusion of the hop-on/hop-off bus, it may help to know that many folks say it doesn't work well in London because traffic is really bad.
In other words, I am doubtful about this pass, but it depends on the users. There's really no shortcut; you need to make a list of the places you want to go and realistically will have time to go, then compare the prices of the covered sights to the cost of the pass. Check each place for family and children's discounts, because they are often available.
LondonWalks offers dozens of different walking tours, lead by licensed guides. They are excellent, and they cost only £15 for adults, £10 for students and £5 for kids 8 to 15. Kids under 8 are free.
LondonWalks