Geoffrey of Monmouth's book was a hit in 12th Europe, but in the present age few would read more than the occasional excerpt of this repetitive account of stylised battles for entertainment. If it has little value for entertainment, does it also have little historical value? Modern opinion has tended to disregard it as a late and fabulous account of events in post-Roman Britain. However, this has been challenged in the most recent years. The text would imply that some larger cities such London, York and Lincoln remained as Romano-British enclaves in the Saxon lowlands, with archbishoprics of the British church at London and York, until not long before the arrival of St. Augustine in AD 597. This is contrary to the traditional reading of the period, but is consistent with modern archaeological findings, and explains Pope Gregory's original plan to have his own archbishoprics at London and York rather than Canterbury and York.
In one respect in particular, Geoffrey's account appears more realistic than more generally accepted history provided by Bede. The latter recounts how St. Augustine threatened the British clergy with war from their enemies when they refused to submit to his demands. A subsequent British defeat in war is then attributed by Bede to the workings of divine judgment. Geoffrey gives a more practical account of how the King of Kent, which now tends to be seen as a base for Frankish influence in the Saxon areas, incited the King of Northumbria and other Saxon leaders to attack the British clerics.
Geoffrey of Monmouth's account will always only be useful as a cross reference to other history or archaeology, but it does imply a a possible different slant to history and particularly religious history in sixth and seventh century Britain.