Is "kinda" a word?

The NOAD reports that kinda is an informal contraction of kind of; it was first used in the early 20th century, and it was originally an American English alternation.

Kind of is an informal phrase for rather.

Wiktionary contains such words.

The entry for kinda (http://en.wiktionary.org/wiki/kinda) includes:

kinda

1. (colloquial) kind of

   I kinda hafta do this right now.

   That's kinda funny.

The Corpus of Contemporary American English (COCA) has 1650 incidences of kinda:

TOTAL SPOKEN FICTION MAGAZINE NEWSPAPER ACADEMIC
1650  172    1023    244      169       42

It is used overwhelmingly in fiction, and the few examples in newspapers and academic texts are almost exclusively in quotations of spoken English.

So, as the other answers have said, kinda is a pretty informal word, not used in formal texts except in quotations. I personally would only use the word in very informal situations. Its 1650 incidences in COCA are comparable to other adverbs, such as besides (1720), tight (1642), and regardless (1607). As to whether or not it is an “actual” word, I think this is pretty clear evidence that it is. As for its acceptability, it is listed in some dictionaries, including Random House and Merriam-Webster.

It is a word in spoken language and used in private letter-writing, but has not yet reached the level of recognition as standard language just as words like gonna, or "of" for have and others. One more generation and it is in dictionaries with a note about usage and in two generations it will be a normal variant.

Authorities at Oxford and Webster do not control or shape the English Language, they only define it. In reality, the collective mind of every English speaker shapes the language. And because some words have gained popularity in some groups and not others, the language has spread out into many dialects. If you've ever read Mark Twain, you might have found his works difficult to understand as he wrote in the dialect of the Mississipi region. Now imagine if all English speakers wrote in their own dialect. We would all have to make a great effort to understand each other. Authorities of the English language basically determine what words and grammatical structures the majority of English speakers can understand, so that every English speaker can read books and other published works without trouble. However, not everyone can understand their English. My mother teaches in an inner city school and many of her students struggle in grammar because phrases like "Y'all goin' to the zoo" sound perfectly fine to them as they are grammatically correct in their dialect. So in short, "kinda" is a word, but not in the common dialect that English Authorities provide. When writing, I would consider to what audience the work is intended for. For example, if I were to write an article about the Packers and Bears rivalry, I would use it because people in the Midwestern United States commonly use it. However I would replace it with a word like "rather" or find stronger diction if writing a formal and proper essay to an English Teacher.