Hey, you've really got(ten) the hang of this, haven't you?
to know all there is to know about XX
"Wow, you're really good at that."
At that = at using smartphones (スマホをつかうこと)
"Wow, you're really good with these."
these = smartphones (スマホ)
"You've really gotten the hang of how to use these, haven't you?"
(何かに上手になってきたということ)
"Heeey, you know all there is to know about smartphones, huh?"
「Hey」を強調するといつもあった感じがかわります。まるで「さすが」だと思っている。