It was certainly seminal when it was printed in the 70s and I'd wager that everyone since then has had to deal with the book, even if they've moved against it. The book fundamentally altered how we talk about the period in and around the fall of Rome - extending it beyond the last Roman emperor in 476 all the way to 750AD, and while not exactly inventing the term of 'late antiquity' (it was the Germans (well, an Austrian)), certainly popularized the translation into English - I doubt the term would be used if it wasn't for Brown.

The book is a revisionist work, and challenged a number of long-held assumptions (going back as far as Gibbon). As well as the above, he argued that the barbarian invasions weren't intent on destroying everything, they were seeking new routes to the Med, and were trying to become Roman themselves, upholding Roman traditions, building in the Roman fashion. So arguing against Gibbon, Brown sees the time period as full of creative expression and new ideas/beginnings, not the decline and fall that Gibbon had suggested (this is why the book is full of pictures of art and sculpture). The book also re-centres the world, not only on the Christian West, but also in the east, including the Persians and Arabs, not just the Eastern Roman empire, and that's also why the time frame is extended to the 8th century with the advent of Islam.

There's a few more topics in it (mostly to do with religion), but the book clocks in at about 200 pages including illustrations, so to do all of that within such a short book is phenomenal. So it certainly is the best thing since Gibbon but there are more recent alternatives to a 50 year old book. It's probably only in the last few decades that anything has surpassed it (Roger Collins' Early Medieval Europe, 300-1000, Bryan Ward-Perkins' The Fall of Rome, Averil Cameron's The Mediterranean World in Late Antiquity, Peter Heather's The Fall of the Roman Empire) but mostly because (at least reading the books) material history has become a/the touchstone for understanding what happened.