Gibbon's Decline and Fall has often been heralded as the "most significant history ever written in the English language", and there is an elegance and scope to him that is immensely rewarding. It is also monumental - the unabridged version is well over 3,000 pages - historians simply don't write on this scale any more. As a scholar, Gibbon is above reproach. He wrote before there was a science of archaeology, and yet his careful attention to detail and use of primary sources wherever available remain remarkably accurate.
For all his attentiveness, however, Gibbon was very much a child of the Enlightenment - and as such took a rather dim view of religion. He viewed Christianity in particular as a dark, corrupting religion, the bitter enemy of progress and free-thinking, and a major contributor to the fall of the Western Empire. This inevitably colored his view of Byzantine history, and led him to dismiss the empire as a "degenerate race of princes" mired in constant intrigue and corruption. This is not to say, however, that Gibbon is not a worthwhile source. Once you are aware of this bias (it is most explicitly apparent in volume 2 and 3 which deal with the Byzantines), the work becomes much more rewarding. As you read, keep in mind that his general position on Rome and Byzantium was that the empire reached its peak during the reign of the Antonines, and then declined - a rather simplistic position that can be seriously challenged in any number of areas. Other more modern works are better balanced - certainly Runciman or Norwich are more accessible, but Gibbon remains a colossus. Vast, expensive, time consuming, and needing to be carefully considered - but indisputably one of the worlds great works.