Not sure about the popularity, but classic none the less. My cousin Dennis is Dennis James. He was DJ growing up. Now that he is a lawyer, we call him Dennis. He was also nicknamed Denny.
Only Dennis the Mennis! haha
But I think Dennis Michael is cute
My nephew is named Dennis and his middle name is Ryan. It's a family name.
In all my years of working with kids (15), I've never had a Dennis, so I don't think it's too popular.
My dad's name is Dennis Frederick (family mn - for my g-pa), and some people call him Denny (mostly his buddies - def. not anyone in my family!)