I had 0 experience with Portillo's before they came to Champaign. But I found out I truly enjoy Chicago-style hot dogs, and the prices are very reasonable.
I have yet to try the chocolate cake shake but have heard it's THE thing to try there. The chocolate cake on its own is pretty good. Edit: I have tried the chocolate cake shake and did not enjoy it -- WAY too sickeningly sweet.
The fries are just okay for me--they're helped if you get the cheese to dip them into.
Once or twice, I've attempted to get into the drive-thru and it's either wrapped around the building with a line out the parking lot nearby, or I've shown up when an 18-wheeler was blocking the entire drive-thru, presumably stocking the establishment.
It's an *incredibly* noisy establishment, mostly from the staff blaring into a microphone for the hundreds of orders going out. If you can tolerate that, then you're probably fine. But I've gone with a family member who is incredibly sensitive to noise and it was not a pleasant experience.
All in all, good food (when you brace yourself for the experience).