好久不见＋了＝OK or wrong? What does the "le" add?
Technically speaking, 好久不见 and 好久不见了 are both acceptable; 了 completes the implicit meaning of "no see" as "have not seen" but can be omitted.

IMHO, the two are more or less interchangeable and it is merely a personal preference of which to use (I prefer the later as well). Nevertheless, from my experience, people usually use the former in relatively formal context. As to myself, I use 了 as a modal particle in this context, as in "Sigh/Wow/Damn, it's been long". I feel that it somehow extends or emphasizes the feeling at the moment. 

On a side note, "long time no see" is commonly believed as originated from Chinese-speaking communities in the States, i.e. it is seen as Chinglish (Please forgive my language).