By Alone-zee's request i trasfer here my comments made in response to identical sub-question in another thread
ну и is an emphatic compound interjection in exclamations expressing negative emotions or attitude, it functions as the English what a...; such a...; so... but only in a negative context, e.g.
Ну и погодка сегодня! - What a terrible weather we have today!
Ну и дурак же ты!/Ну ты и дурак! - You're such a fool!
Ну ты и загнул!/Ну и загнул же ты! - You're so exaggerating!
Ну и видок у тебя, Шарапов! - You have such a terrible look, Sharapov!
- Could it be that this "и" is a particle for emphasis that often precedes a verb?
I doubt that. ну и goes not only with verbs and rather to a lesser extent with verbs, and it doesn't follow some previous statement for confirmation or reinforcement, it is perfectly autonomous and self-contained.
Check these examples of usage with a verb:
1. Ну ты и пишешь! (a comment about quality of handwriting) - You have such an awful handwriting! (characters are either too small, illegible, ugly, don't meet some arbitrary requirements or something else etc)
VERSUS
in a dialogue:
2.
-Зачем я это пишу? - Why am i even writing this?
-Тебе сказали писать? - Have you been told to write this?
-Да. - Yes.
-Ну / Вот / Ну вот ты и пишешь. - So that's why you're writing.
Note that in the second example the phrase doesn't end with exclamation mark, it would be inappropriate here because it's not an exclamation.