Gibbon grades Marcus Aerilius Antoninus's successors using the last of the Great Roman Emperors as the benchmark. Each emperor thereafter is respectively a degree away from Marcus Aerilius in ethics, morals and values. Guards, friends or family readily dispatch those that live as the model good citizen. The author, however, keeps his hope in finding a hero. He includes the Empire's adversaries, such as Attila and Alaric in describing wanton virtues for a good ruler.
Written two hundred years ago, the language is far from contemporary. But, if you are serious about learning Western History and Culture, The Decline and Fall of the Roman Empire is a must read. Make the commitment and read Volumes 1, 2 and 3. From there, you will gain a better understanding when reading other epics on history, such as The Discoverers by Daniel Boorstin.
More importantly, you may say that you have successfully walked with Gibbon through nearly 500 years of history!
A word about the text. Everyman's Library reprints the famous J.B. Bury edition (Bury was a famous Irish historian who wrote a well-respected History of Greece), which is close to 100 years old (it dates to 1909). If you're reading Gibbon for a history course on an undergraduate or post-graduate level, you should probably read the more recent David Womerseley edition, which is available in a three-volume Penguin paperback (with, unfortunately, unreadably microscopic type). The hardcover edition was remaindered recently, though, so you might find it on Amazon secondhand.
If you're reading Gibbon for pleasure, however, the Everyman's Library edition is the one to get. The individual volumes are just the right size, and the text is large enough and clear enough to be read easily. The text is complete, which is not always the case (some fancy editions -- the Folio Society's comes to mind -- tend to cut back on the footnotes).
Gibbon makes great bedtime reading. Take him slowly, and don't rush. Keep your eye on the footnotes -- some of the best and snarkiest stuff in Gibbon is discreetly hidden in the footnotes (in one of my favorite early footnotes [in Chapter IV] he mentions the giraffe, "the tallest, the most gentle, and the most useless of the large quadrupeds."). If you decide to push on to the second three volumes (Chapters 39-71), be prepared to be patient, because there are some rough spots. It might take you a while to get through it (my last reading of the entire work took me 26 months), but Gibbon is more than worth the effort. Which is why I've just started reading him again -- for the fifth time.