I,too have sailed on all three. Fixed seating at dinner is a dealbreaker for me, but Celebrity was doing an 11 day cruise out of and returning to Charleston SC, so I cared not about anything except the convenience of driving to and from the pier. Would I sail Celebrity again? No. Too big and fixed seating. My expectations, however, were exceeded and it did represent great value.
My review of our 28 days on Insignia are attached.
www.luxurycruisetalk.com/ubb/ultimatebb.php?ubb=get_topic;f=17;t=000097#000000