This cable car crosses the Thames between the Excel Centre and the O2 building. The walk from the Excel Centre took about 10 mins but the Royal Victoria stop on the DLR is slightly closer. It cost us £3.30 each for a one-way trip using an Oyster card (it is £4.40 otherwise). We went on a cloudy but dry Sunday in January and had to queue for about 5-10 minutes to get onto one of the cabins. The cabins would take 10 people but only about 6 were getting on each time, giving everyone plenty of space.
The views of the Thames and O2 were good but the other buildings visible close by are definitely not the best that London has to offer. Perhaps on a sunny day more of central London would be visible. North Greenwich on the Jubilee line is the closest underground and we reached this in about 10 mins, walking past the O2 on the way.
We enjoyed the trip and thought it worth doing but would not bother to repeat it. I think if you had not been on a cable car before, or had young children with you, it would seem more exciting.