We are fans of the Marco Polo. It's not perfect, but we wouldn't keep booking if there were serious problems. Everybody has their own experiences and views, and whilst I don't doubt some of the moans about the ship have a basis in fact, some of them are nonsense. Some people just like to moan - you see that on other travel sites as well.
Anyway, our most recent cruise was a short break to Amsterdam, Rouen and Antwerp. Due to traffic problems on the canal linking Amsterdam to the sea, and hence missing a tide, we got an extra stop in Honfleur. This was a bonus (though inevitably, some people were moaning!) Rouen had courtesy buses, the other city centres were all within walking distance.
Embarkation couldn't be smoother (has been all four times) and getting on/off ship for shore visits also very efficient. Staff are universally friendly and helpful - generally not British, but we like that - it seems like you're on holiday before you leave Tilbury! Some posters have complained about the English language skills - we saw one passenger repeatedly saying to a waitress "oi'll avan'arf" instead of asking for a small beer, then telling his companions what an idiot the girl was for not understanding English. Smile and be pleasant to the staff, and they'll be pleasant to you - it's not difficult! Beer incidentally was £3.10 a pint - that is cheaper than my local pub.
Meals - some of the criticisms are valid. The self-serve restaurant can be crowded when the weather isn't good enough to sit outside, but is better if you can bear to wait a few minutes, rather than joing a queue before they start serving. The buffet is pretty good - I'm a fussy eater and I never had problems. Breakfast can get a bit dry if you're not there when things are first put out, but that's often the case in hotels, and these people are trying to feed 800 passengers. Dinner in the Waldorf Restaurant is great - varied and tasty - when you see reviews describing it as school dinners or works canteen, you do begin to wonder!
Shore excursions are expensive, but you don't have to go on them, and we generally don't.
Public areas are fine - always being cleaned and decent decor. Gym is well-equipped, given the profile of the passengers. Cabins maybe aren't as well soundproofed as a modern ship, but we never had any problems yet (touch wood!) Cabins in category 12 have more storage than I have at home! Also two lovely big windows.
Entertainment is generally excellent - I recently went to a West End show featuring so-called TV stars, and the dancing was definitely no better than on the Marco Polo. some shows are better than others - the Queen tribute is not my favourite, but others (eg ABBA, rock'n'roll, classical concert) I could watch again and again (and have).
Daytime activities are a bit amateurish at times (competitions etc) but people enjoy them and they're not compulsory! Lectures are varied - usually art, gardening etc, but on this cruise the lectures were by a former cold war nuclear submarine commander and were very popular.
Finally a comment about the passengers. They do tend to be older (we are in our late forties and are among the youngest - my wife does get mistaken for crew - best not to wear tan trousers and a pink shirt!) You get some moaners but on the whole people are there to enjoy themselves and everybody gets on ok. Age range tends to be younger on the shorter cruises - average probably 60s/early 70s.
Overall, don't be put off by the moaners!