It is very casual, yes; similar to 'dude' for men. Although 'dude' is sometimes used when talking to a woman, 'dudette' would be more accurate (but this term is seldom used).
As far as negative connotations, it may depend on the context. The term 'chick' may be offensive to some, depending on how it is being used and by whom etc..
That said, we typically use the word 'chick' when describing someone (especially strangers). It is very informal and may be considered disrespectful to some.