I loved Mercury and Galaxy. DH and I did our last two cruises on Mercury before he passed away in Aug. '08-- Jan. '07 to the Mexican Riviera and Jan. '08 to Aust/NZ.
We did a Feb. '05 Galaxy cruise to the Panama Canal (partial transit and 4 other western ports. His sister took his place on the previously booked S. Carib cruise on Galaxy in Jan. '09. I miss both ships. Love the M-Class (5 on Summit, 1 on Connie and 1 on Millie)and am cruising for the first time on Silhouette in Jan, again with my SIL. Never been on Century, but wouldn't mind trying it sometime.
I remember chuckling at a comment a newbie made about how HUGE Mercury was. She couldn't believe it when I told her it was really a lot smaller than most of the newer ships.