Yes Casino is certainly not a classic, but streets ahead of most directors out there.
One of the problems in gangs to me what his use of music, he is usally second to none in this department..see Goodfellas, but it spoilt the expereince for me before it even started .. one of the probelms there were others.
Saying that this man made Mean Streets, Taxi Driver, Raging Bull, Goodfellas, The king of Comedy and The Colour of Money. All fantastic so I am not going to write him off yet although I do take your point his last good film was a while ago.