Yes, we did use the 3 day London Passes for our trip to London in April. We were traveling with 2 kids and 4 adults. For us, since we wanted to see a ton of sights with the kids it was worthwhile. We were there for 7 days so on the non London pass days we saw the free sights. If memory serves me right, here is what we saw with the 3 day London pass:
Double decker hop on hop off bus - don't bother unless you do it early Sunday morning, because otherwise you just sit in traffic
Tower of London
Tower Bridge
Kensington Palace
Westminster Abbey
Boat ride down the Thames
Churchill War Rooms
Arsenal stadium tour - request of the 12 year old boy, don't bother if you are not a soccer fan
Tea and scone at the Victoria and Albert - surprisingly lovely experience
So, in order to make it worthwhile, you have to pack in several sights each day. If you'd like a more leisurely trip, this wouldn't work. If I was traveling on my own, I probably wouldn't have bought one, but the math did work in this situation. My favorite sights were actually free - the Victoria and Albert Museum, the Museum of Natural History, and most of all Sir John Soanes house in Holborn. And best of all I just like walking around through Leicester Square, Covent Garden, Westminister area, Hyde Park, Marylebone and St. Johns Wood. My favorite sight that charged admission was St. Paul's Cathedral which isn't on the London pass, but you can get two for one tickets if you are able to use railpasses and meet the very picky criteria for that.
If you do decide to get a London Pass, buy it ahead of time when they are on sale and pick it up in London at Leicester Square. Feel free to PM me any questions.