Did it this week.
Just went to ANSES with my DNI, they have a few copy machines there, so if you don't want to bother getting your DNI xeroxed somewhere else...
Took me about 1 hour beginning to end. Got in, went to the Information desk (1st cola), got a number referring me to desk 6 (2nd cola), where they typed my data into their system and then I had to wait about half an hour or so in the main hall (3rd cola - and last), until they called me by name. I had to hand out my DNI copy and wait for another 5 min or so until they called me again and gave me the CUIL.
After you get it at ANSES the first time, you'll be able to do it online as well. They told me it takes 48 hours to have your data properly set in their system and then you can have it generated online.
Hope this bit of info helps.