The problem I’m seeing with these kind of sites is that the designers are trying to compound many pages into one, and aren’t realizing that with that many different requests for mostly things like images it creates a massive load time and an annoying blankness if you get too far ahead of the loading.
However, the one that @sl1dr posted loads perfectly. It’s minimalistic and uses less images that are smaller. One of the challenges I think anyone that would be wanting to design a website like this would be minimizing the load time and making it flow properly. Now when I say that, if you go to some of the websites with this effect the effect doesn’t move smoothly when you scroll down and thus makes it less impressive.
Nizo is a good example of a site that doesn’t do the effect properly, because the animation is choppy and it really brings down the wow factor for me. Then with Ala, the time it takes to load the site is far too long and by the time you’re ready to scroll down the images below it still aren’t finished loading.
Even with the challenge of learning how to create this and do it well you still have a lot of elements to consider before being able to make an amazing version for yourself. Although when you do get it right, you can be sure you’ve accomplished something great.
Also here is a great side scrolling example: http://yessbmx.com/
I personally love this one, it’s high quality and it loads quickly. It also creates the effect well and even with a few kinks still is one of the most amazing designs I’ve seen in a while.