The observation wheel turns slow enough for people to embark while it is moving. A complete turn takes about thirty minutes. Thanks to the construction of the glass capsules on the outer side of the rim, the passengers have a great 360 degree view over London.
Many famous landmarks are clearly visible, including Buckingham Palace, St. Paul's Cathedral and the Houses of Parliament. On a clear day you can see as far as forty kilometers (25 miles).
Make sure you get your tickets in advance, lines can be very long, both the lines for embarking and for ticket purchases. It's less crowded at night when the views are even more spectacular.