I have had both good and bad experiences with them. There is one local to me and I have gone in person to by my current DSLR and kit lens, also my Sigma 70-200 f/2.8. I was looking for a telephoto and didn't want to pay the Nikon price for that lens when the sales guy turned me on to the Sigma. Special ordered it in, threw in a B&W UV filter for free and gave me a nice little discount on top of that. He was flirting, that was fine by me.
I've also ordered online from them a few times. One transaction was so so. I had multiple items in that order and some were "special order" so they kept the whole order waiting for those things to come in stock. I got fed up and told them to cancel the order and I'd buy elsewhere. They then rearranged the order and shipped out the available stuff immediately and then sent the remaining stuff when it arrived at no extra cost.
I will always compare prices with McBains because they almost always have the same items for cheaper or will price match. They gave me a wicked deal on my Sigma 10-20 and on my SB -600, oh and the Nikon 50mm f/1.8 was cheaper there also.
I have just ordered the new Sigma 24-70 f/2.8 IF EX DG HSM from Vistek as well, it was cheaper than McBains or any other place in Canada and it was shipped out that day! Pretty happy about that. If something says "special order" go elsewhere or you could be waiting weeks. Which is unacceptable really. Why should it take weeks to order something in??
So I would say that price wise, it's almost always better to shop elsewhere, but once in a while you can get a better deal at Vistek. If you can only find that item at Vistek, call McBains, they will order in anything you want.
And if you are not opposed to letting the salesman flirt with you, you can definately score some great deals.
ETA: I think Visteks free shipping starts at $400.