I'm writing from the point of view of someone that has cruised with Disney a few times and loved it (but it was too expensive) and did quite a few Carnival cruises (cheap, but inconsistent quality). We had heard that Norwegian fits somewhere in between these 2 options, and based on our first NCL experience on the Sun, we agree. While the entertainment and service are no where near Disney Cruise Line's offerings, this was certainly overall a step or 2 above our recent experiences on Carnival.
The most notable advantage that we observed on the Sun over our last few Carnival cruises (even on small ships) was that the Sun never felt crowded. There were always places to sit and relax, whether at the scenic outdoor buffet on deck 11 or in the shows, bars or other public areas like the library or atrium. Also, contrary to any Carnival ship we've been on, the Sun has enough dining choices, both free and premium/paid, to keep you fed at most times of the day. There are ample free options including multiple buffets, 2 free dining rooms and several premium/paid dining options. We never had much of a wait getting seated in the dining rooms, and the buffets were never very crowded, even on embarkation day. As others have noted, the food in the dining rooms is, in large part, repeated, so don't expect much creativity. However, the food was almost always very good, and the dining room and buffet staff were always attentive (both improvements over our last 3 Carnival cruises).
On the downside, since this is a smaller, older ship, it rocked...A LOT. To be fair, we were traveling through some rough seas, with waves from 5 to 8 ft at times. But by far, we felt it more on this ship than any other recent cruise. Also, due to a poor design, cigarette smoke from the casino can become overwhelming at times, and not just in the casino, but also in the Atrium area as the smoky casino air tends to waft throughout midship on deck 7 (note: this is due to be fixed in an upcoming dry dock). Also, another minor annoyance/quirk of this ship is that the older layout can be confusing at times, as you have to travel through venues to get from one end of the ship to the other on deck 6 (ie: you have to go through the Bliss lounge, Windjammer bar, and Photo Gallery to get from the Atrium area to the theater (Stardust lounge).
We stayed in a balcony cabin (9030). While the cabin was showing its age a bit, it was sufficiently roomy, clean and comfortable.
Embarkation and Debarkation were very quick and easy.