Yes, people can look into the celebrity suite from the stateroom balcony next to it. Just keep the curtain down on the window closest to that balcony.
If you have a Sky deck suite (floor 9) you may want to consider moving down. We found the noise level from the Resort deck very loud. It sounded like a kitchen preparation area right above our suite, constant clanging and banging.
The bathroom tub (no shower enclosure) is somewhat awkward. You have to step very high to enter the tub. The tub bottom is elevated from the bathroom floor which makes getting out a challenge.
We did the Millennium 10/4/02 and loved it. The shows, the food, the service, the entertainment, the Olympic restaurant, all were excellent.
Their were some blemishes though:
1. We had dining table for 10. The A/C at the table was so cold that 6 out of 10 requested new tables. We didn't because we felt sorry for our waiter and assistant waiter.
2. Last night of cruise, plumbing problem caused black water in sinks and showers. My wife was taking shower at the time.
3. Butler service.