The question I've posed in the title is not an easy one to answer. It all depends on what you are looking for. Marco Polo has many plus points, and more than a few minus ones.
This is my attempt at a balanced personal opinion based on one Northern Lights cruise. It will, no doubt, repeat much of what can be found in other reviews, but also add a few other insights.
Let's start with embarkation: if you were to design a location and condition of a cruise ship terminal that least matches up to the world-class city that is London, then Tilbury Docks is it. It's a dour, miserable, industrial disgrace, whether joining or leaving a ship. That said, the people and level of organisation of Cruise & Maritime on arrival was superb, with friendly, informed staff making the whole process easy and quick (including parking the car in the convenient, even if overpriced, under-cover area).
We were taken straight to our cabin, an upgrade from the one we had reserved, which proved to be on a par with those of the other ships we've sailed on, ie. perfectly adequate and very clean (we only ever book modest cabins), and situated conveniently for the public areas, but more of this later.
What is the overall feel of the ship? She's old, but that is not apparent when below, as the cabins, corridors, and public areas have a modern feel and everywhere is really clean. On deck it is a different story: the pool deck and the smaller tiered aft decks above it are fine with functional furniture and well-worn laid decks (there is no substitute for laid decks; they have such a good feel underfoot and are naturally non-slip). As soon as you move forward however, and even before the laid decks are replaced by painted ones, you feel you are on an old ferry rather than a cruise ship, and this feeling is reinforced by the horrible-looking passenger foredeck which has a surface that can be very slippery, and the working foredeck complete with open bins for warps and a pair of derricks. Much of this is unavoidable in a ship that has been around for nearly 50 years.
Marco Polo carries about 800 passengers plus crew, and it feels like it. Only when half the passengers are at dinner do you feel as if you have a bit of space; the public areas and restaurants are not really adequate to service this many people, especially on a cold-weather cruise where the pool deck is more or less out of bounds. Having to have two sittings for restaurant meals with pre-determined seating is also not my idea of fun; who wants dinner at 6.15pm?
What can I say about the food? It's a strange mixture; the galley is capable, and sometimes does produce really lovely dishes. It can bake a cake too (unlike certain other ships) but too much of the food is bland and rather poorly presented with the layout in the buffet restaurant designed to produce long queues at busy times. Overall, definitely not up to the standard of the other two (smaller) ships we've been on.
After being used to friendly, smiley, Filipino crew elsewhere, I was bothered about the standard of MP's waiters, cabin staff etc. I needn't have worried. With very few exceptions, the Eastern European and Asian crew were lovely and we got to know several of them quite well by the end of the cruise. They were smart, professional and friendly and made a real difference to our enjoyment. The entertainment staff were also great. Whilst we had no desire to sample much of what they were offering, we found them really enthusiastic and often genuinely talented: helped in no small way, no doubt, by Richard Sykes, the Cruise Director. At first sight, he appears a bit of a 'cheeky chappy' and OTT, and I suppose he is, but that guy has so much talent too. We watched and listened to him lead a passenger choir who learnt and sang a couple of four-part harmony songs, and his musical and coaching abilities shone through. He is a real asset to the ship: so it's a pity he has to deputise for the Captain too - yes, really. Not in sailing the ship, which the captain appeared to do superbly well (unless one was looking, one remained completely unaware of berthing at any of the ports we visited, so smoothly was it done, and rough weather was apparently handled very competently) but in making noon announcements of ship's position etc. You really need a captain to engage with the passengers - it's part of the cruising experience, but this was just one example of an apparent disconnect between senior staff and passengers. Compared with a smaller ship one felt processed, just a number, and rarely were senior staff seen out and about talking to passengers.
Life aboard can be quite varied but it depends what interests you. If you want to play Bingo, watch and play game shows and attend a range of musical shows and late-night discos, you will have plenty to occupy you (an important note regarding the latter entertainment which takes place in the aft-located Scott's Bar: don't even think about booking an Amundsen Deck cabin anywhere near this bar as there are sufficient numbers of ill-mannered, inconsiderate passengers aboard to keep you awake until at least 1am by ignoring the ship's signs urging other routes to be used to and from the bar. This problem persists because the ship will not take a more assertive stance on enforcing peace and quiet on the two cabin corridors involved).
If you are fascinated by the places you are visiting and crave the help of some expert lecturers to educate and entertain you, then you'll be sadly disappointed. What talks there were had nothing to do with the itinerary, and the ones I went to were poorly presented. There is certainly a wide-ranging demographic on MP and I suppose they have to cater for the majority but we regarded this situation as a major deficiency.
If your socialising and entertainment is best lubricated with a drink or two, then you will find the bar prices mostly acceptable with just a few quirks (like regarding dark rum as a premium spirit). The house lager is bland but drinkable; more than can be said for the house white wine which was dreadful (we actually bought a glass in two different bars in case we'd had a bad bottle, but no, it was execrable both times). The bar staff love mixing you a very fair-priced cocktail or an equally affordable iced liqueur after dinner.
Excursions are a different matter with many, justified, complaints about over-pricing, but MP is not alone in cruise ship rip-offs ashore.
The ship is pretty comfortable when sailing; she has an easy motion, and even in very rough seas feels quite predictable apart from the occasional stagger when she falls into a trough. She is much smoother and quieter than the other two ships on which we have cruised.
And, finally, all of these comments must be set in the context of the low pricing that Cruise & Maritime often offer. Their two-for-one deals place them way higher in value than other small and smallish ships. However, even though extremely value-conscious, I would happily pay somewhat more for fewer passengers, better food, and appropriate lecturers.