Just wanted to share our experience today.
With Airbnb check in not until 2pm we had planned on checking our bags at Termini then spending some time at the nearby National Museum before picking up bags and taking the subway to Airbnb.
Line was huge around noon on a Friday. There is a non-obvious (read: we missed it and just went to end of line) ticketing machine/system for getting a turn at the counter (think taking a number at the grocery meat counter but on a little automated machine). Left my gf in line with bags while I went to find a bank ATM (found downstairs close to where you connect to metro) then came back to find gf had not moved much. Finally figured out we needed to use the machine. There is a more expensive express option which we felt wasn't worth it for a short bag check (probably an hour at best by this point).
We ended up just forgetting the bag check and grabbing a casual lunch downstairs in the station with our bags in tow.
Hope this helps someone else!