I cruise with NCL because their itineraries are port heavy with very good excursions. If you use the $50 per excursion perk, it is very affordable. Embarkation and disembarkation are very easy. Entertainment is diverse and fun. Crew seem to enjoy their jobs (not a fake atmosphere of hospitality). Speciality restaurants provide an upgrade from the regular fare, which is adequate but nothing special. There are always lots of fresh options if one wants to eat healthy. I have gotten to the platinum level, and plan on cruising with NCL in the future. I am a solo traveler, and NCL is safe, affordable, pleasant, and relaxed. I cruised a "high end" line last year, and really did not like the formality and presumption of the ship. The cruise experience is a matter of taste, but for me, I enjoy the happy, fun, relaxed, and diverse experience of Norwegian cruises. The Spirit is an older, smaller ship, which I also like. The ship is clean and well maintained.
More than adequate for a solo traveler. It is on the smaller side, so I would not think it would be comfortable for more than 2 people. My cabin steward was super.