"I really appreciate it" is correct and usable in written English. You can use this phrase to express gratitude in response to someone's help or kindness. For example, "Thank you so much for helping me out with this project. I really appreciate it.".
I really appreciate it when people say "no" to me.
"It was uncomfortable, but I really appreciate it," Leetch said.
I really appreciate it the second time around.
"There's a level of toughness that defines this team, and I really appreciate it".
I really appreciated it".
I don't think I really appreciated it until I moved away to London.
I thanked him and told him I really appreciated it, but I'd seen enough.
Being a terminologist, I care about word choice. Ludwig simply helps me pick the best words for any translation. Five stars!
Maria Pia Montoro
Terminologist and Q/A Analyst @ Translation Centre for the Bodies of the European Union