I was lucky enough to fly from LHR to JFK at the end of July in 2003 - during the run-down to retirement in the October.
As has been said the seats were about economy in width, but with more legroom. Leather and very comfortable, as you would expect.
The cabin is compact - only 4 abreast - and the headroom is just adequate for most (the very tall have to stoop).
A couple of interesting points are that the windows are much smaller than usual (a passport nearly blocks one out) and, once cruising at Mach 2,you have the odd feature of a "warm" wall next to you from the kinetic heating of the aircraft.
The food and wines were superb and, although there is no video, there is an audio system. However, the flight itself is sufficient entertainment to keep the headphones in their bag.
The most unique feature, as already posted is the speed. Just under 3500 statute miles (5600 km) in 3 hours 20 minutes.
The aircraft I flew on (G-BOAD) was probably the last to visit SYD as it completed a RTW charter late in 1999.
In 1979/1980 BA and SQ operated a service to Singapore via Bahrain with a flight time of 9 hours. When SQ took delivery of their 747-200s that could do it non-stop in 13, it put paid to the venture. Indeed, even then, the writing was on the wall for supersonic air travel.
Fantastic aircraft. Still unrivalled.