A number of papers have been written discussing visual magnitude estimates in comparison to photometry. Multiple studies have shown that visual magnitude estimates compare very favorably with instrumental photometry in terms of sensitivity and consistency. As with all data, care must be taken to understand what visual magnitude estimates represent, what their bandpass is, and when their use is appropriate.
The following papers, published in the Journal of the AAVSO and elsewhere, provide information for researchers interested in using visual magnitude estimates in their studies of variable stars.
Links:
[1] http://www.aavso.org/publications/ejaavso/v36n1/77.shtml
[2] http://www.aavso.org/publications/ejaavso/v31n2/128.shtml
[3] http://www.aavso.org/publications/ejaavso/v27n2/97.shtml
[4] http://www.aavso.org/publications/ejaavso/v27n1/65.shtml
[5] http://www.aavso.org/publications/ejaavso/v26n2/151.shtml
[6] http://www.aavso.org/publications/ejaavso/v19n1/33.shtml
[7] http://www.aavso.org/publications/ejaavso/v16n2/118.shtml
[8] http://www.aavso.org/publications/ejaavso/v8n1/26b.shtml
[9] http://www.aavso.org/publications/ejaavso/v7n1/10.shtml
[10] http://www.aavso.org/publications/ejaavso/v6n1/4.shtml
[11] http://www.aavso.org/category/tags/aid
[12] http://www.aavso.org/category/tags/analysis
[13] http://www.aavso.org/category/tags/calibration
[14] http://www.aavso.org/category/tags/data
[15] http://www.aavso.org/category/tags/photometry
[16] http://www.aavso.org/category/tags/visual-observing