Having sailed with Costa 5 times and with another booked for next year...I happen to prefer the line to the others I have tried (P&O, Olsen, NCL, Cunard).
As a solo, they cannot be beaten for price...especially on longer cruises. Half the fleet has dedicated solo cabins with no supplement and the ships that don't have solo cabins I pay a maximum of 30% supplement.
I love the variety of itineraries....I hate the Caribbean, been there, done that I never want to return. Middle East, Far East & South America are my favourite regions and so far Costa have never disappointed on itinerary in those regions.
I also love the cosmopolitan feel on board the ships...having sailed with predominantly British and/or American passengers on other lines...mixing with other nationalities & cultures is a wonderful experience...you can learn so much about people and even if they don't speak perfect English, you can still hold a conversation if you try hard enough.
The food on any cruise line is enormously selective, often due to where you come from and what you feel that you should expect....I have no problems with the choices or quality on Costa...one of my favourites is their lamb curry, which is so wonderful I asked for it specially for the last night of my most recent cruise...it was to die for...perfection on a plate!
I personally feel that people often try a line with too many preconceived ideas...totally wrong way to go about it IMHO.
Go with an open mind and remember that you are not sailing on a British or American orientated cruise line and just enjoy yourself.