Murf I read The Scar first, before I knew it was part of a series (which I have since completed). It, and all the others, stands alone just fine.
Ryan Anderson I read The Scar first, and I'm glad I did: I liked it more than Perdido Street Station!
Lisa Boone I read Perdido Street Station first and I don't think it is necessary to have done that to enjoy this book. It did give me a lot of background on some of the locations and different "races" of folks in The Scar, but I think you would be fine reading The Scar on it's own!
Eric Miller I read them out of order and it did not bother me. It was obvious the story took place in an established world, but it was clearly laid out and explained. Pretty amazing books, hope you like them.
Herli de Menezes Yes, it's possible to read it before reading Perdido Street Station. There are few references to New Crobuzon and to some events which occured there.
Arved I read them in order, but the only things I got from it was a deeper understanding of the world - because things that are introduced in perdido street station are introduced more extensive there, and the (very slight and not at all important) connection between the two book's protagonists.