The in-laws are utterly utterly obsessed with cruises. We have been roped along on two P&O cruises with them (big birthday celebrations which they have generously paid for). I can appreciate they are the sort of holiday which could suit older people or those who are less mobile. The range of facilities on-board are better than your standard all-inclusive, but without the hassle of planes and transfers etc (unless a fly cruise).
We found the experience quite stifling e.g. be back to the ship at this time (or it leaves you), eat at this time, put your luggage out at this time. Being in port for such a short period of time was also incredibly frustrating. Literally only about 4 hours in some cases. We spent most of the time in queues to attractions, or heading back to the ship (to allow good time for delays).
Food was sub-par (compared to our expectations following all the rave reviews) and probably equivalent to "vintage inn" pub quality food. It all seemed to be meat and two veg options, apart from the speciality restaurants. The buffet was very repetitive and although looked huge, was just repeated across multiple stations. It was standard "all inclusive" quality (with a heavy emphasis on roast dinners) and nothing special. Both me and DH felt very uncomfortable with how the staff are paid and treated. 99% of the staff (apart from the entertainment staff) were from India, Philippines, Sri Lanka etc and we felt they were probably not being treated very well compared to other staff members. It had a bit of a Downton Abbey (but less cheery) vibe which we found very very uncomfortable.
We also found the "sea days" very boring and although it looked like a plethora of activities were scheduled, many of them were thinly veiled sales pitches e.g. "explore skin care with x products" or "explore jewellery with x products". We spent time reading, walking round the ship, watching movies or planning our next meal. We were on a cold north sea cruise, so swimming in the pools or sunbathing wasn't that appealing. The spa looked lovely, but very expensive.
So yeah, we will probably get roped along again...it is difficult to say no when it is for big occassions and someone is generously paying. We enjoy spending time with family etc, but we found ourselves a bit flummoxed as to what the fuss is about.