What is the status/reputation of the University of South Africa (UNISA)?
I offer the following evidence from credible and official sources against doing a PhD in Computer Science at UNISA:


Low graduation rates:  17 doctoral students graduated from the College of Science, Engineering, and Techonology (which includes the PhD in Computer Science degree) in 2010, 2011, and 2012 combined. Compare this to the 99 doctoral student enrollments five years earlier in 2005, 2006, and 2007 (combined). Graduation rates are expected to be somewhat low for distance learning students, but rates this low are a bad sign. (PDF source)
Inadequate supervisory capacity: The school admits that "many [research] areas in the School of Computing have reached supervisory capacity" and says that "long waiting lists started forming due to lack of supervisors," which is another worrying sign.  (Same source)
Very, very low research output: Research output is arguably the most important indicator of a reputable PhD program. On this page, 6 professors in the School of Computing (which offers the PhD in Computer Science degree) are listed as having openings for PhD students. I looked up the Google Scholar and/or DBLP profiles of these 6 (for those that had them) and consulted personal pages of the rest to get a sense of their research productivity. 

Averages across two years (2012 and 2013), the number of publications per person per year was 0.58 on average (range of 0-1.5)
Of the 7 publications I found for these professors in 2012 and 2013, 3 were in conferences or journals with the name "Africa" in the title (i.e., not international venues). So the average publication rate per person per year in non-local venues was 0.33.