Gibbon, recognising that he lacked the time to continue his work in the same detail as before, does miss out most of several centuries of Byzantine history. Whilst this is a shame, there are some interesting diversions, perhaps most notable chapters on Genghis Khan, Tamerlane and the splintering and then fall of the caliphate.
The writing style remains sometimes tricky to read, but it's merely highbrow rather than Shakespearian. I was pleased to see that although Gibbon's dislike of Byzantium generally continues he was full of praise for a certain emperor (I shan't spoil it) who suffered a tragic fate.
Towards the end of the work we are taken back to Rome for various shenanigans involving popes and we hear of the final tribune of the Eternal City.
It ends with a couple of appendices, the latter dealing with a list of the Roman emperors and the former with a few pieces of Gibbon regarding the various updates and publications of his work. I must say that I was pleased to hear him refuse to correct or change his earlier work as it would offend those who bought the earlier version, and it brought a wry smile to my face when I read of his decision to retire.
Given these are the final 3 volumes of a 6 volume work you will either have disliked the first set, in which case you would be an idiot to buy this, or you will have enjoyed it, in which case I recommend buying this (although you may wish to skip the religion section).
Overall it's a masterpiece depicting the transient fragility of human endeavour and the construction of mighty edifices which the erosion of virtues and passage of time crumble into dust.