The archaeological evidence doesn't really accord with Pirenne's statements about it (and far more contradictory evidence has been found since he wrote), he's got some weird ideas about Islam (Muslims did trade with Christians and with other People of the Book without major issue for most of the medieval period, so far as I'm aware), and there's an anti-Semitic undercurrent to a lot of what he says about Jewish merchants. Of course, its very neatness should raise alarm bells.

It's an appealing idea in its neatness, and in its emphasis on the dependence between Western Europe and the areas around it, and the prose is astonishingly well-written for a first draft (Pirenne died before the book was finished, and it was ultimately published by his son). In other words, without Mohammed, no Charlemagne. From this new climate came feudalism, the medieval Church, and the Holy Roman Empire. It was only with the emergence of Islam, which ended the unity of trade and of cultures around that sea and shifted the political and cultural centre of European gravity northwards, that Late Antiquity came to an end. European civilisation was still essentially Mediterranean and centred on the Mediterranean Sea. Pirenne's claim is essentially this: that Romanitas-Roman culture, trade, social order, etc-survived the disappearance of an emperor in the West and the Germanic invasions.

The Pirenne Thesis is one which has been much debated since this book was first published (as Mahomet et Charlemagne) in the 1930s.

Pirenne mohammed and charlemagne