Advance dining reservation are made online on your account on the O site by accessing Manage My Cruise. You should do it as soon as possible once the window your assigned according to your stateroom category opens.
If you are in a suite, PH or above, your butler can help you get additional reservations, depending on availability. You also check your self at the dining reservations desk or with the maitre d of the restaurant. I don't know the success rate as we've also been happy with our guaranteed reservations. But I understand you would have to be flexible...late times and/or sharing.
We think the GDR is good and have no hesitation eating there. Some folks may not agree, but food is subjective. The menu rotates over 14 days, but there are some regular items. We've always been able to get a 2-top, although we sometimes had to wait 10 minutes. The GDR opens at 6:30 and folks are line up to get in.
Some people prefer the Terrace for dinner but we prefer being served rather than going through a buffet line.
Included drinks are in cans, and blended coffee drinks are available (also included).
Movies are free, as to up to date, our experience is minimal. Perhaps someone who watched them can pipe up.
Toiletries are upscale and in bottles, not pump jugs.
We haven't used room service, except for a hot breakfast, and the selections are excellent. If you are in a PH or above, you can also order from the restaurant menus, including the specialities during meal times and your butler can serve you course by course, if you want.
The is no upgrade bidding.
I'm sure other will chime in if they have other experiences or want to expand on their views.