Before going to morocco, every guide books i reard warned about Casablanca the big bad city.
Actually, casablanca is one of my favorite moroccan city after Rabat.
Ok, that's modern, ok that is smelly, polluted and really, i mean..really noisy (and i live in london so i should be used to noise pollution)..
DO VISIT THE HASSAN II MOSQUEE, IT'S THE ONLY MOSQUEE opened to non-muslims in morocco. This mosquee is beautiful and visiting it remains one of my fondest memory about morocco.
Also, casablanca is full of cheap and excellent restaurants (not really friendly service though).
Casablanca is pretty safe compared to Tangiers but BE CAREFUL IN THE OLD MEDINA.
I had a pretty bad experience there, do avoid to go in the old medina , because there is nothing interesting to see anyway.