the "Insignia" has not yet entered service with Oceania, hence there are no reviews, yet. The ship is currently in drydock where several million USD are spent on improving her.
I sailed on her Oceania sister ship "Regatta" last September, and it was a superb trip. Oceania's food is among the best at sea, and the ship is very comfortable.
Don't worry about the weather in the Mediterranean in May. I really prefer spring and fall over the hot summer months. Temperatures are often around 20-25°C.
Tom Ogg has a very comprehensive review of the "Regatta" posted here
Also, my Dutch colleague Bart de Boer has a superb review with many pictures on his web site
My own review will be published on the internet in March.