The London Eye bills itself as "the world's largest cantilevered observation wheel." The entire cycle lasts just under 30 minutes. There are excellent views on the way up and on the way down, but know that you will reach the peak height of 440 ft. in 13-15 minutes.
On clear days, you'll be able to see the central business district, the parliament buildings and both Charing Cross and Waterloo train stations. You can buy a map for £1 ($1.58 USD) that opens into a perfect circle showing the panoramic location of London landmarks. One side is a day view, while the reverse is a night view.