Yes, it is a common expression you can use to greet people you haven't seen for a while. It may sound a bit strange, because it doesn't follow the rules of English grammar, but it is widely accepted in spoken English. You could also say: Oh my gosh, look who it is! Well if it isn't my old buddy (friend)...! I can't believe it's been...years!

When I learn English at my school, I will find that the grammer between English and Chinese has lots of differences, but "Long time no see" is a funny one. It is just like the Chinese "好久不见" . It even matches with every words, "好“ means ”good“ in most of times, but also has the meaning of "very" "久“means ”long time" "不“ simply means "no" "见“ means "see" And when our school teacher told us, and we just have the same qusetion like you do because it doesn't seems like the NORMAL English sentenses we had learnt before. BTW. Thank you for revising my little essay~ that's helps a lot~ Hope we can learn from each other ^^

I personally use that expression quite frequently! It would be my go to. The context I would use that in would be usually an old friend who I haven't seen in a while. This is because the expression is quite informal. It can also be used in a semi-formal situation however. The expression can also be used in a joking manner. For example, someone who you saw just 2 minutes before whom you bump into again. You may exclaim in a joking manner "Long time no see!". Other expressions you could use are as follows: "It's been a while!" (It's been a while since you last saw them) "My gosh, look who it is!" (This is quite commonly followed up by "Long time no see!") I hope this helps!