We cruised the "Romantic Danube" itinerary with Viking in April 2014 with three adult children on the Freya. Freya is one of the new Viking longships and a definite step up from their older vessels. In addition to the very light and airy Scandinavian design throughout the vessel, the new longships have an elevator for passengers with mobility issues, an indoor/outdoor lounge area, and shuffleboard + putting green on the top deck, The regular rooms (I have not seen the suites) are compact but very well laid out. Ample closet space though another few hangers would have been nice. Lots of drawer space as well as space under the bed to fit luggage meant that everything could be tucked out of sight. The housekeeping staff keep the rooms spotlessly clean. The balconies are big enough for two chairs and a small table but we generally preferred to go up the public areas, with soaring glass windows inside or the lounge chairs outside. One complaint I heard from other cruisers who did want to use their patios was that the boats were often docked beside each other, so one's balcony view was a stateroom on another ship.
River cruise ships are much smaller than ocean going liners, so the experience is more intimate but also more limited. There is no fitness room, pool, sauna, theatre, etc. There are no children's activities (and indeed few to no young children on these trips.) There is one bar and essentially one restaurant. Entertainment in the lounge is pretty basic: a piano player/singer throughout the day (same person) + some evening local entertainment brought on board, albeit limited based on the size of the dance floor area in the bar. I would guess the average age of the cruisers to be about 50. If this is what you are expecting, you will be pleased; if you are comparing it to a Caribbean or ocean cruise, you will likely be disappointed.
The restaurant was very good. Serving staff were experienced and provided extremely attentive service. If one chose to sit in the same area and get the same wait staff daily, it was not long before they knew your name and your preferences. My daughter dislikes onions, and after the first dinner the waiter brought all of her food with the onions removed without her asking. Viking has bumped up the dinners a notch on the new ships to be more like fine dining. The portions are slightly smaller but the presentation is upscale. One could ask for more but we found the portions filling without making us feel "stuffed". Each night had three new choices for appetizer/entree/dessert and a cheese course based on local cheese course. If none of those appealed then steak/salmon/Caesar salad were always available. Breakfast was a full buffet with omelette bar (+ one could order eggs benedict, pancakes, etc. fresh from the kitchen). Lunch varied, depending on whether one went to the restaurant to eat or was lucky enough to get a table in the Aquavit lounge (indoor/outdoor) on a nice day. We enjoyed eating in the lounge but the tables are limited and all round, so going with a party larger than four is difficult. The dining tables seat six, with a few larger tables for bigger parties.
The port excursions are a combination of included tours with some optional tours that could be purchased separately. The included tours were mainly general overview/bus tours of the city in question, taking in the biggest sites in a couple of hours. The optional tours (usually on the other half day) were more of a variety. The tours are well explained in the pre-cruise guide and at the nightly briefings. The guides were all local and a bit hit/miss for the novice guides so early in the season. Viking runs a "leisurely" tour for those a bit slower on their feet but it was hard to tell the difference. What they need is a "spritely" tour for the younger cruisers. If one is not the patient type, the waits for everyone to disembark, get on the bus, figure out the headsets, etc. can be frustrating. Of course, one can also dispense with the tours altogether - the concierge service for arranging your own activities was very good.
We had flown into Budapest a few days before the cruise started and I think from now on we will generally do a pre-cruise extension, which speaks more to the generally lousy/unreliable airline service these days. It's far more pleasant not to spend the first day or two just de-stressing from getting to embarkation! This particular cruise spends the first two days in Budapest so it is a bit less stress-inducing. Having said that, the concierge service is really top notch at working out issues with late flights, lost luggage, etc.
Of course, some aspects of the vacation are beyond the control of the cruise company: bad weather, low water, lost luggage, late flights. The experience is partly what you make of it. We have experienced all of these problems and still had a good time.
We find all of the Viking Cruises to be a reliably good value. Their website is great! If you take the time to read the information available you will be well informed with few surprises.