Lukas - please note that these proofs are hosted by the printer at their location, for instance the links to the first two are hosted by me in my office and the last link is hosted on a box at Compose's office. The benefit of this to the printer is NO HOSTING FEE - and as you see by the statement "a little slow" that it is very effective under most any kind of hosting environment.
Magnus - sending out 150 dpi PDF files might be ok under a lot of cases - but consider a case like the one below. Here we have tons of small detail and lots of important "Legaleze" that must be carefully read. I think you would agree the zoomable proof is a much nicer method of viewing it. I am certain your existing "Web Proof System" is likely at least an order of magnitude more expensive than using TIFF Pager.