I liked Casino. It's definitely not my favorite of his, but it's still great. I dunno how you mean toned down - if you mean gore, it's very bloody, way bloodier than Goodfellas and The Godfather. If you mean toned down as far as themes and quality, then I guess I would agree. I wouldn't trade Sharon Stone's role for nothing, though. Very annoying, but a great downward spiral. And it seemed like both De Niro and Pesci weren't the monologue type anymore.
Still, Casino had its moments. Like when Pesci kills someone with a pen. Or when Frank finally gets his revenge on Pesci by beating him with a baseball bat.