I am currently updating a computer at work to El Capitan. After downloading, I started the installation almost 48 hours ago and it hasn't finished yet. I know (by experience and through reading various threads) that El Capitan is pretty slow to be upgraded but 48 feels dangerously too slow.
Note that the machine has 500GB of storage and has been used in the past mostly for storing things and for running various processes coded in C, C++ and R.
How it looks like
The screen is blue and empty with only the below loading sign at one third of the height from the bottom. I did not stare at this computer for the past 48 hours of course, but I would tend to think that it reached this screen in a matter of minutes and then jsut got stuck there for the past 48 hours.