I used the Oxford manuals and found them a bit hit and miss to be honest - I found them difficult to learn as a student but having passed the exams I do find myself using them often as a reference and reminder tool.
I thought the General Navigaton and Meteorology manuals were very good but then others such as Air Law seemed to have been cobbled together just to fit JAR knowledge objectives without too much thought being put into it.
I only used the Oxford manuals myself so I don't really have a frame of reference to compare them with. But as others have said they do go into more detail than needed usually.
I wouldn't use them for distance-learning as one could easily get bogged-down with learning the more unnecessary details - If in a full-time ground school the student would be guided to the more important areas by the Ground instructor so I think they would be better in that scenario.