Yes, this is a somewhat casual phrase that is generally used among friends. Grammatically, it's awkward, so I wouldn't use it as an example of English grammar, but it is a commonly accepted and understood phrase.

Can anyone else say "REPETITION"?!?! haha, how many times do you guys wanna give the same answer? it's already been said that it's informal, you don't need to say it again! Cherry: " It has been long time that I have not seen you " " I didnt see you since long time " etc are both gramatically incorrect. Aisha: Of course you wouldn't say it to someone you don't know....!! For that you would say "Never seen you before!" Just for clarification, YES you can say it and YES it is informal but even if you say it to your boss, it's OK! I do!

Hey Kuniklo! ^^ It's perfectly normal to use it with friends, but it's informal if you use it with someone you don't know. The correct way to use it is: "It's been a long time since I've seen you." "I didn't see you in a long time." GOOD LUCK!! ^-^

Yes its an appropriate expression in oral English. Originally is wrong, this expression was invented by a famous Chinese offspring actress, she said this sentence first, because she just translate it straightly from the Chinese expression:好久不见." But since then American are widely accept it.

To be honest with you, "long time no see" is grammatically similar to Chinese. haha Chinese people say "Hen Jiu Bu Jian" They are of the same meaning and grammatical order to "long time no see". xD Isn't that interesting I've heart English people use "long time no see" on the street in the U.K. So i guess it is a good expression.