Hi,
We used online check-in last year.
We had already reserved our seats on-line and then checked in the day before before driving down to Manchester (getting the same seats we had reserved). I think you need to enter the passport numbers for all travellers.
We also printed out the confirmation.
We still turned up three hours before the flight but we didn't need to wait in a long line to check-in although the bag drop line didn't move overly quickly.
Certainly worth doing as it doesn't cost anything and we did end up with the seats we reserved - unlike our return journey where we were shifted back a row.