Westcott, Photoflex and Lastolite are all known producers of quality stuff. I personally know nothing about Interfit, which you should take in no sense other than that I don't know anything about them.
I have bought quite a few Photoflex items over the years and they're all what I've expected, and if I'm not mistaken all of it is still with me. I don't have personal experience with Westcott, but a couple of my friends use their stuff.
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!