I've booked cabins on the Riviera deck in two different occassions, and, let me tell you, I just love it. Unless I book a cabin with a balcony, or book a guarantee ocean view cabin, I will never book a cabin on a different deck.
The Riviera deck is very quiet, and if your cabin's in midship, you hardly feel the ship moving. I was taken aback when I read your post where it says that someone said that the Riviera deck is the slum of the ship, nothing could be farther from the truth; as far as rats and cockroaches are concerned, I've never seen the critters running up and down the hallways, or in any other place for that matter.
All the decks are the same as far as cabins are concerned (unless you book a suite, or something). The only downside I find with the Riviera deck is that you have to climb the stairs to get to your muster station for the mandatory muster drill. You can't use the elevators for that. Unless you just have to be close to where the buffet is, and can't handle the walking, I find absolutely nothing wrong with saving some $ and booking down there.
We've been "Titanicsized" and when we think of the lower deck, we think "steerage," and although that might've been true aeons ago, it doesn't hold true now, and it doesn't matter what deck you're on, you receive the same service and attention from crew members.
Go ahead and have yourself a wonderful, beautiful cruise.