We cruise only RCI and Celebrity and I can say there is not a whole lot of difference. I might have said Celebrity had a slight edge in the food dept. But we were on the Jewel in June for the Brit. Isles/Nor. Fjords cruise and the food was the best I've had on either line. The service was perfect and we had a super great time. Of course, we always have a super great time no matter what the ship. But truly, the food was the best we've had so far and we never use the "specialty" restaurants.
If there are problems on any ship, it doesn't translate to tainting the whole line with the same brush.
For the person who is reconsidering which ship to take to Alaska--I would highly recommend the Summit. We sailed her in Aug. '03 and it was our first Celebrity cruise. We didn't make the choice because we were traveling with friends who got us in on a group package. We had sailed Explorer twice and wondered how we would like a smaller ship on another line. But we loved the Summit and later fell in love with Galaxy and are expecting to fall in love with Mercury next Jan. We also loved Adventure and Jewel. I don't think you can go wrong with either line and it's nice that they are "sisters" so to speak.
I did some research on who goes to Aus/NZ and at first it looked like we would have to try Princess, but now it's been announced that Mercury will start the Sydney-Aukland and vice versa itineraries late next year. That means our next 3 cruises will be Celebrity: Mercury to Mexican Riviera (booked 2 weeks ago), Galaxy to the Med.(we chose this because of the 5-day land tour to Italy that RCI didn't offer and the smaller ships are usually cheaper) and Mercury to Aus/NZ. I would try Princess if my favorites did not go somewhere, but now it's not necessary.
1/02 Explorer E. Carib.
1/03 Explorer W. Carib.
8/03 Summit Alaska cruise/tour
2/04 Adventure S. Carib.
2/05 Galaxy Panama Canal
6/06 Jewel Brit. isles/Nor. Fjords + 3 days in London
1/07 Mercury Mexican Riviera