It's of great fun.还是It's great fun.两个表达都可以吗?

问题描述:

It's of great fun.还是It's great fun.两个表达都可以吗?
fun这个词可以用 "be of 名词" 这种形式吗?还是直接当成一个不可数名词表述?

It's of great fun.