Fotiman - 2:22 pm on Mar 23, 2010 (gmt 0)
I just started Opera to see this for myself. I saw the "Removing Previous Version" dialog briefly, then another dialog which had a checkbox to "Automatically install updates without notification from now on" (which was checked by default), and buttons "Download and Install", "Remind Me Later", and "Help". I chose to Download and Install.
Near the end of the download, I got an error that said:
"An error occurred during download (could not validate file). Unfortunately, we are not able to recover from this error automatically."
I chose to "Resume", which seemed to do the download again, this time with no problem. I ran the install and when it was done, it hadn't changed any of my file associations. My default browser (which is not Opera) still opens .htm and .html files when I double click on those file types.
So my experience was not as bad as yours.