I can report back about my experience at Helsinki now. It was great. This is my new favorite airport to transit in Europe.
Arrival. Passport control took a little longer as the officer in Rome had not scanned my passport to show that I was leaving in April. A few minutes conversation with officer in Helsinki was required. It still was a total of about 10 minutes. They were not using separate machines but did the finger printing etc at the passport officer booth. I asked why I needed to do it all again since I had already registered in Rome in April. They told me that EES is being so unevenly implemented across Europe that for now they are requiring every time.
Departure: I had a long layover, but knew that I only had 45 minutes in September so I decided to time how long the process was gate to gate. The entire process, walking to the passport control, standing in line for a few minutes, processing, and then walking to the next gate took just a little over 20 minutes.
You won't have the tremendous lines that are sometimes referred to in other airports. The airport itself is clean, modern, and pleasant.