I used it for our Med cruise this summer. I was able to choose seats for our United flight out on United's site. The flight back is United operated but Lufthansa and I could not select seats for our family on either site.
I got a non-stop to Rome on a red eye at the same price as booking directly with United. We leave 3 days ahead of the cruise, but with the red eye, arrive 2 days ahead.
After being on a Med cruise with Princess and being stuck on the ship in Istanbul because of the 9/11 attacks, I was glad we booked our flights with the cruise line. They bent over backwards to get us on a charter after just 1.5 days, quicker than my brother in law who was stuck longer in Kansas.
I understand things are different now, but with a European cruise, I'm hoping this is our least risky option when it comes to logistics/delays/cancellations.