First one thing: do not, on any account, get the abridged version. If I could take one book to a desert island, it would be this one. That's because it is extremely long, and every word of it is worth it.
Gibbon's Decline and Fall of the Roman Empire remains as relevant as ever. And this is in spite of its hugely ambitious scope, treating of the history of the Roman and Byzantine empires (both considered Roman by Gibbon) from the end of the 1st century AD to the 15th. Gibbon is a modern historian. He is shrewdly selective of his sources, judiciously reserved, and coldly analytical. He differentiates between proximate and ultimate causes. He has a humanistic but impartial point of view. At the same time, he is an 18th century Englishman. While this is reflected in some of his opinions, such as that the extinction of republican freedom was what determined Rome's decline, it makes them no less valid and often the more interesting; it is hard to imagine anyone today being able to treat the early Christian controversies with the same tact and humour, for example.
And Edward Gibbon wrote like an enchanter. I read somewhere that his style was an inspiration to Churchill. No wonder. Every line of this tome of perhaps a million words is a delight to read. You will laugh out loud. His thought is clear and convincing. And there are simply magical moments, such as when he produces that mythical animal that appeared in the Roman circus, an animal no one in Europe has seen since then... a giraffe. Or the dissertation on whether Europe remains at threat of invasion from the Mongols.
The Decline and Fall is full of telling anecdotes, and yet it always holds to a general picture. It is filled with detail and colour but never loses the reader. It is packed with events, and it offers discussion of longer trends - notably those that participated in Rome's decline and led to its eventual fall - political, religious, military, economic. And it is even more impressive when one thinks of the modern tools its author did not have at his disposal, in particular archaeological and numismatic. Approximately half of the book is dedicated to the Roman Empire proper, up to the late 5th century. This is where Gibbon is at his strongest, his research the most thorough. The rest deals with Byzantium, touching heavily on European history up to the fall of Constantinople, and has a broader sweep. His work ends with a description of Rome as it looks today (i.e. in the late 18th century).
I finished reading my copy (after several happy months) in Rome itself, in a little place with a view of the Pantheon. If you have the luck of being able to do that, you will never forget it.