I've been shopping with them since about 1998-9 and haven't ever had problems with them getting the items out the door on time, when it's an item sold and shipped by Amazon. I tend to avoid buying anything on Amazon that isn't sold by them though as I have had problems when trying to buy from other companies they act as a storefront for.
Come to think of it, I've started buying less from them now that their searchs turn up so much crap sold by third parties through Amazon...but when I do order, Amazon seems to handle their side of things very well.
UPS on the other hand is a different story... A couple weeks ago I ordered a cheap Wilton/Columbian bench vise [1] and Amazon had it to UPS later that day. According the UPS package tracking dealy it sat in Cincinatti for 3 days before making it's way to Houston. Pretty much every time I've had anything shipped by UPS where delivery estimates are a range, like 3-5 days, they always sit on it long enough to just make their deadline, never the earlier of the two estimates. I don't like UPS at all. I have the same UPS irritations when I order from Lee Valley too.
Interestingly, when I was still living in Anchorage I'd get packages from Amazon shipped USPS priority faster than I get UPS packages down here in the lower 48. Much faster. Back at one point in time Amazon used to let you choose your carrier instead of just picking your shipping priority but I've noticed they've stopped doing that. Which I find irritating since I've always recieved better service from the good ole USPS than I ever do from one of the commercial carriers...
Anyway, enough bitching for now...
Cheers,
Josh
-------------- [1] kinda crappy Chinese import but servicable for my immediate needs, I'll get my $20 out of it before upgrading I'm sure.