I can see they'd be a bit marmite, but I quite like them.
Went on one pre-Dc, with another couple. We went out of Mombasa, and down to/through/round Madagascar. It was lovely - smallish ship, time spent with friends, good visits to islands we wouldn't otherwise travel to.
We did another with the dc a couple of years ago. We have 3 dc, then aged 9 (with severe ASD), 7 (with AS) and 18 months. It worked for us for many reasons. Dd1 loved having the continuity of the ship, whilst visiting different places. Dd2 enjoyed the kids club, and getting to know a large-for-a-7-year-old environment which she could roam around safely. Ds didn't care where he was 
It worked for dh and I because we had entertainment on tap, never more than 5 minutes walk away - kids clubs,, silly games and activities, swimming, ice skating, shows, roller blading, basketball courts, small soft play room, but also an opportunity to get out a bit in port and see a small glimpse of other stuff. The buffet worked for us, because we could all find something to suit, without compromise.
I would happily do another cruise, but am not exactly a die hard convert - since then we've had a few other family holidays. The dc all loved it and continually ask when we are going in another cruise (dd2 has her heart set on a Disney cruise)
In an ideal world, no I'd not do them either. In my head, family holidays are all about being off the beaten track and exploring. Hill walks, safaris, nothing commercial or organised. But that's not our life - autism means regular, organised and routine works much better. So given we need that, a cruise worked well enough. Still an opportunity to visit a zoo, or aquarium, or science discovery centre, but each in a different country.