This was our 2nd cruise this year - our first was with Princess in the Caribbean, with a balcony - and we wanted something not too expensive, which is exactly what we got.
We originally booked an interior cabin, but having worked out the actual size, decided to upgrade to an outside. We made a good choice, as having seen the size of the interior ones on board, I don't think we'd have been happy. Quite apart from the fact that they're really tiny, there's no natural light, so would have felt extremely claustrophobic. Many people do cruise very happily in interior cabins, but it's not for us - no matter how cheap.
There have been several reviews saying that the food on the Costa ships is mainly frozen - even the salads. Well that's rubbish. Quite apart from the fact that lettuce (and there was a lot of lettuce) doesn't freeze, every salad we had was obviously fresh. However, almost every dish is marked as having one or more ingredient from frozen. This seemed particularly true of the seafood, which my husband said he was quite disappointed in. Overall, the food was of a reasonable/good standard, and in fact some (not all) of the main courses were very good - particularly the beef and the soups. One thing though - at breakfast, the toast is dreadful, and they really should take some lessons on poaching eggs - didn't have a decent one all week. The fresh fruit every morning was lovely, as were the pastries.
We ate in the main dining room on early sitting. We did try the buffet - but to be honest, I don't go on a cruise to eat off plastic plates in a canteen, and have to say the food there wasn't really to our taste. Nothing wrong with it - in fact the pizzas were the best we've had on a cruise - but you don't get the lovely presentation with buffet food.
We did try the Concordia restaurant (20 euros pp extra) on our first night, as we'd been mistakenly allocated 2nd sitting (maitre d' changed this for us from night 2 onwards), and it was very good. However, the vibration from the engines, although it didn't bother me, really annoyed my husband. We asked the staff if it's always like that and they said yes. Also seemed a bit odd that the Concordia restaurant was open to and above the buffet restaurant, so as we were eating our meal, we were serenaded by a crying baby from the buffet below.
I've also seen several comments about the announcements being in 5 languages. Yes, it can get a bit annoying, but if you book an Italian ship, what can you expect. We cruised with MSC 2 years ago, and it was exactly the same. If you think about it, they could just do everything in Italian, so in some ways they're offering a better service than English speaking ships.
One of the problems with 'budget' cruises, is that they have to make their money somewhere - and a lot is made up from the drinks and drinks packages. On Princess, we paid about $20 for an unlimited soda package (we don't drink much alcohol), but on the Concordia, it was 40 euros plus 15% service for just 20 cans. I got through 40 cans in the week, and my husband refused to pay on principal, so just drank iced table water.
The excursions were OK - but again slightly more pricey than the Princess. The organisation was also a little hit-and-miss, but nothing went majorly wrong.
On our trip, there were about 3,200 passengers, and it felt very crowded indeed at times - goodness only knows what it would have felt like with a full complement of 3,700. That said, we never struggled to find a sun bed, but it was a little difficult to find somewhere quiet to read sometimes.
As far as the shows are concerned, they were very good, considering they had to appeal to a multitude of languages, and varied from a tribute singer to a magic show (which was surprisingly good). However, for us, there was too much time between eating and the shows - 1st dinner sitting was 7.00, but the first show wasn't until 9.45. 2nd sitting was 9.30, and the 2nd show was at 11.30. Would have been much better for us if the 1st show was at 9.00.
The English speaking hostess (Angela, a Spanish girl) was fabulous, always trying so hard to please everyone, and very friendly indeed.
Overall, for what we paid, we got an excellent cruise. But if you want Princess or HA standards, you won't get that on a Costa. With cruising, you definitely get what you pay for.