This was our first Transatlantic, and our first Celebrity. Overall, I would say it is a notch below Oceania, but one up on HAL and two on Princess.
Food is important to us, so the following comments would not apply to those who enjoy a "pile it high burger dinner".
The main dining room - very disappointing. Very low food quality, which excellent service could not compensate for. As bad as Princess. Tables for two are very badly positioned in the corridor. The usual attempt to rush you out in 30 minutes.
Top deck cafes - what you'd expect. Not really for us.
Bistro on Five - again excellent service, but a very limited menu. The emphasis on crepes is a mistake, as they are not of a high enough quality. Only two salads (with appalling dressing - best avoided). Good soups that change daily. Very nicely fitted out. The formula overall does not work, as it was half empty most days. If I were Celebrity, I would use this space to put in an Asian speciality restaurant which would be packed and which they could charge more for.
Tuscan Grill - again excellent service, especially wine and from the one actually Italian waiter. Menu way too limited. The frozen fish did not appeal, so you have to like steak. No chicken, pork, lamb etc. One or two creamy pasta dishes were ok, but the chef is not Italian and it REALLY showed on the lasagne. Again, if you like your pasta sauces without wine, garlic, or herbs, then this would not matter for you, but we expect Italian food in an Italian restaurant. Perhaps clients should be given a choice.
Ocean Liners - again outstanding service, both managers excellent, and a wonderful place for flambees. A more varied menu than the Tuscan Grill, and higher quality food. This was our refuge. It was quickly clear who was who, as we saw a lot of fine diners again and again. Slight shame they don't use the piano there, one guest took to playing for the rest of us which made the atmosphere even better.
Overall - the extra charge for the speciality restaurants is high, but we had little choice given the poor food in the main dining room. If you are not a foodie, then it is not worth paying a lot. The "Ultima" package which allows unlimited access is sold as including lunch, tea and dinner, but be aware that is a bit of a con as there is only one lunch in TG and OL, and only one tea (ON THE SAME DAY!) in the entire 15 day cruise. You need to calculate your options very carefully.
DRINKS - As others have commented, wine prices are high, higher than Oceania which seems particularly ridiculous, and a good 50% above HAL. Martini's is a great bar, though a walkthrough feel can be off putting. We like our wine so we bought a package, but again this is a bit of a con. You are of course restricted to glasses only, and even the upgraded package meant hardly any wines were available on the OL menu. That $12 cut off is cunningly low. This is only worth it if you are a heavy spirits drinker who drinks every day. Having said that the price of wine by the bottle was astronomical, so really you either drink average wine or pay through the roof.
ENTERTAINMENT - Similar to other cruises, but overall enjoyable. Cruise Director John was very good, as was the Mercurial Tyler, the activity manager. The dancers were better than the singers, but they put on a great show. Individual entertainers were perhaps a little downscale, but still very good. The speakers were excellent - particularly Nigel West. Way above normal. Internet access is a problem on this ship, and the system they use is particularly user unfriendly. The Internet staff were the one area of the Constellation where the service fell down.
CABIN - average, but ok. Similar to HAL, below Oceania. A lot of guests were talking about poor internet service, and no TV reception if you were in a lower cabin.
CROWD - good age mix, and I would say a fun bunch. More varied than other cruises, though being a Transatlantic, no children.
PORTS OF CALL - very interesting, and the variety was very appealing. 8 days at sea non-stop were a challenge, and it would have made sense to break it up with a stop somewhere west Atlantic. The time changes were oddly handled. Have never been on a ship where ship time was different to the ports of call, and there was real confusion in Madeira with old time, new time, and local time (not helped by the tour tickets being printed on shore time). Finishing in Amsterdam was a step down. Dreadful weather, most restaurants seemed closed for some reason, and the flight we took on to France with Transavia was a definite mistake. $300 for an extra bag! We wished we'd got off earlier.
OVERALL - a good ship and an excellent crew. If food quality could be raised then this would be a winner.