We LOVE the second deck porthole rooms on the classic ships. We LOVE it. We stayed there for a two week PC, and have stayed there for several 7 day cruises. We like the back third of the ship, close to the stairs/elevators. It is quiet, the view is great, and the access to just about everything is perfect. One floor up to lounge, atrium, restaurants, etc. The elevator to go to the top decks is right there. I can't think of a single drawback. You might think the second deck is low, but consider it is much higher than many yachts of the world, and they have great views. We also usually prefer porthole rooms if the price is really different, because we can easily find a quiet deck chair to sit in and watch the view (as well as people), and pocket the extra $$.
We LOVE the second deck on the classic ships (back ⅓ of the ship).