They were certainly a great side. Beating Australia in the Ashes 3-1 in the year in question was a major achievement, considering that the Australian team was a rather intimidating one that contained the likes of Neil Harvey, Richie Benaud, Arthur Morris, Keith Miller, Ray Lindwall and a young Alan Davidson. They also went undefeated for what was until recently the second longest stretch of 14 series between 51 and 58.
However, there are a few things that reduce them as a side in hindsight a bit, including the rather spectacular way their run ended when they were thrashed 4-0 in the Ashes, and went on to lose to Australia again in the next Ashes contest after winning a few series against the other international sides of the time. Because of this they really only dominated test cricket for a few years, and during that stretch they were seriously challenged on several occasions. Fair to call them a great test side, but when you stack them up against the Invincibles, the West Indies of the 70s and Australia in the post-2000 period, could you really call them the greatest of all?