My wife and I have been cruising for several years. We've gone on 6 cruises together and been at sea for over 40 days. This was our first NCL, in the past we've cruised on Royal Caribbean, Celebrity, and Princess.
The Norwegian Spirit is an older ship, but is still well-kept. It is far smaller than the newer mega-ships at about 2000 passengers which makes it a nice mid-sized ship. We had an interior room on Deck 10, probably the best deck to be on (unless you're prone to seasickness, then go to Decks 4 or 5). There is some interesting quirkiness throughout. The ship was originally built for cruising in Asia for a different line as the Super Star Leo. Although the ship has been refit recently there is still some remains of the old name as well as the fact that it was built for an Asian clientele as much of the signage was still in Chinese and English. Also you can see where some areas were repurposed but not fully re-themed. The most obvious was Cagney's Steakhouse, which still had terra-cotta statues outside it and a Chinese themed decor. The ship goes with Norwegian's Freestyle Cruising model, which at first we were skeptical out; however by the end of the trip we decided that it was great, especially for Europe. On a Caribbean cruise that isn't has port intensive or with a large group, the Freestyle model probably wouldn't work as nice.
Service: Overall the service was excellent throughout the ship. At the top, the Cruise Staff and Hotel Director were very visible throughout the ship and often stopped and talked with the various guests. I was the hotel director get pulled aside while walking throughout the ship several times and he always stopped to talk with the guests. The stateroom attendants were excellent, they always seemed to figure out right when we were gone and took care of our room during those times. We never came back in the afternoon (even earlier in the afternoon) and didn't have our room made-up. Also on sea days when we slept in, they never knocked or came to our room before we had gone out. The dining room staff was mixed. In the Garden Room (mid-ship main dining room) our service was excellent every time we ate there and the waitstaff in the buffets and other casual bars and restaurants were equally great. The bar staff throughout the ship was very good, they always checked on you, but it didn't feel like you were being bombarded to buy drinks constantly either. The only average service in dining was in the Window (aft main dining room). There we had a variety of service from average to excellent. It seemed a little more disorganized than the other dining venues.
Venice is an amazing city that must be visited. We stayed an extra night pre-cruise which was worth it. If you stay you need a hotel around San Marco (St. Mark's) square. It's in the middle of everything and a great starting off place.
Istanbul is an amazing city. Highly recommend either a guided tour or independent research before going. It isn't marked well from the cruise dock as to how to get to the various sites. They are redoing the port now so that may change in the future.
We had advice from a friend who lived there are several years, so we had a great idea of where to go and how to get there, but I could see someone not familiar having issues getting around without a plan of where to go.
Barcelona was an amazing city. We had an extra day planned after disembarkation to visit the city. There was so much to see and do that we didn't have time to see everything but we saw some great sites.