We've sailed on all the Celebrity Millennium-class ships, and can't say enough good words for them. My wife loves the warm salt-water Thalassotherapy pool, and we both find the dining and entertainment options more than adequate for our needs.
We aren't big onboard spenders - tips and duty free liquor are the bulk of our end-of-cruise bill - yet we don't feel at all deprived. Though tempted to try Qzine, we passed, as the MDR service and food was great.
We've made it to Elite status with Celebrity, and cannot pass up the continental breakfast with complimentary coffees and cocktails, or the evening happy hour and other events.
Big window, with a substantia sill for storing small items, books and papers.