I visited this site in 2006 when I didn't think a lot about WHSs yet and hadn't seen many sites. Even then we found it surprising if not ridiculous that this site should be a WHS. The site consists obviously of two sites: Suso the upper monastery is tiny but has a certain charme with visigothic and mozarabic elements that I would certainly appreciate today though they have hardly any OUV on their own. Yuso, the lower monastery, on the other hand, lacks any special character or beauty. There are dozens of more interesting monasteries in Spain alone. 

Even in the nomination the claim for fame does not mainly refer to the architecture: "It was here that the first literature was produced in Castilian, from which one of the most widely spoken languages in the world today is derived". and: "The Spanish language, a principal work of human creativity, was born, in both spoken and written forms and in both prose and verse, within the eremitic communities of San Millan". I think this accumulates several strange assumptions: The fact that the oldest preserved written sentences of Castilian are found here does not prove that this language was created here or even written down for the first time. And to call Spanisch a "principal work of human creativity" is hard to believe. It is one of many Latin dialects and as such not more valuable then any other Roman language. That fact that it was later spread around have of the globe doesn't make it more creative or ingenious then any other language. Or we should have at least one site per country concerned with the birth of a language. We see, even then the arguments for inscription were sometimes humbug. 

The camino francés is really a treasure trove for romanesque and early gothic architecture. If you travel there Yuso and Suso is an easy and short visit but certainly not a highlight.