My wife is mobility impaired and usually uses a wheelchair and so a lot of this report relates to the accessibility aspect of our two way Atlantic crossing on the QM2.
We checked in about one hour early and that did not cause a problem. Embarkation at Southampton was easy and efficient although the postcode given on our e-tickets seemed to refer to a park in the middle of Southampton rather than the port. We had pre-booked parking and from arrival in our car to getting into our state room took about 40 minutes, the longest queue being security. We were directed to a special check-in area to one side of the terminal with low desks suitable for those in a wheelchair.
Getting from quayside into the ship involves a lift and a bridge similar to that in airports with quite a few difficult to negotiate ramps. Staff (at both ends) on the way out seemed to have been instructed not to help those in a wheelchair manage the ramps and it was annoying to have to struggle to get the chair over the ramps with someone in uniform standing watching from a few feet away.
Standard cabin, space acceptable, lots of coat hangers, air con / heating worked well. Bed very comfortable. Bathroom bigger than we had feared. We did not use the balcony - who does in the Atlantic in November ! We have no complaints and would happily take the same cabin again