This was our third cruise and, again, there are certain aspects we love but a lot that we don't. The obvious plus is getting to various locations without having to pack and unpack. How can you beat that? We also like that you can be fairly flexible: if you want to sleep in and just go in to town in the afternoon, you can. If you are an earlybird, you can skip the evening entertainments (which we don't find all that terrific) and get an early night and be off the ship as soon as it docks. Tours are a bit less flexible. Of course, being on your own is the most flexible.
We definitely didn't care for NCL. This was our second cruise with them and we almost definitely won't use them again. They nickel and dime you to death. There are a couple of dining rooms that are included in your basic fare plus the buffet. And then there are specialized restaurants that charge a per person cover charge. Of course you don't have to go to these restaurants, but it's nice to try something different, as the same food gets a bit tiresome after a week.
The size of the cruise ship is also something you will want to look at. These NCL ships carry more than 2,000 people and the new one carries even more. We preferred Oceania with 600 people on the ship. Strictly a personal choice. The smaller ship will offer fewer restaurants and perhaps fewer choices in entertainment. Oceania was certainly more expensive than NCL.
The excursions from any cruise lines are quite expensive. We don't usually use these. During this cruise, we used only one, in Northern Germany, as we felt it would be difficult getting anywhere interesting on our own. It was quite good. But in other places, particularly when we were docking in a city, we used the Hop on Hop off buses that are usually available right at the dock. They're not dumb: they know where the business is. Some cities you don't even need that: in Copenhagen we just walked in to town. We'd been there for 2 weeks on a home exchange several years before so felt there wasn't anything we really needed to see; we just had lunch and walked around the city a bit.
We did arrange for a tour of St. Petersburg through a local (Russian) agency instead of through the cruise line. This was a big savings and, I think, a better experience. See the "Experience" about St. Petersburg in 2 days.
Friends who had never been on a cruise before signed up for an excursion in almost every stop and were quite disappointed in several. They had a "crazy" tour guide on one.