I heard this last night in a television advertisement during which someone associated with the TV show “nip/tuck” was discussing the joy of the show satirizing itself.

Problem:
Although the American Herritage Dictionary says that “fun” is sometimes used informally as an attributive adjective, “fun” traditionally is a noun.

Explanation:
The speaker used the word “very” as an adverb to intensify an adjective, but the word “fun” traditionally is a noun. Use of “fun” as an adjective is still noted in dictionaries as an informal use. Although such expressions as “a fun time” [sic] started to appear in the 1950s and 1960s, this use of “fun” as an adjective should be avoided, especially in writing. If “fun” is properly used as a noun, then it requires an adjective, not an adverb, to modify it.

Solution:
“which is much fun”