Alternative dining is interesting. I remember on my first cruise with RCCL, the "alternative" Johnny Rockets dining, and believing it was worth it (looking back, not sure if their burgers differ much more than regular burgers on the ship, but it's been quite a few years).
Recently on the Carnival Liberty, I went to their alternate dining, Harry's. $30/pp charge for a 5 course meal.
The food? Not much different than the main dining. I had the filet mignon, which was HUGE. The next night in the main dining, they had the same thing (smaller) but same quality. To me, the large filet was way too much, especially with all the courses.
The biggest difference to me was the service. At Harry's, they made the salads and such in front of you, and the staff was very attentive. In the main dining, we had mixed service (we had your time dining and sat with the same server most of the time, except for the last night), so that was a benefit. Was it worth it though? I'm not too interested in going back. It's also interesting that with Harry's, all throughout the cruise they were advertising if you booked with Harry's on certain nights, you would receive a complimentary bottle of wine with your dinner. It seemed like not many people on the ship were interested in alternate dining (when we were there there were 5 other couples, one of which fought with the server and left before their first course)
If I were to do alternate dining again, I'd like to see something different than what I could order in the main dining. Maybe a hibachi or a mexican cuisine; just something other than a typical steakhouse.