Hmmm, "scaling" WordPress is an incredibly simple thing to do. Caching and a modern stack is all you need. If you view the source of all of their featured clients and scroll to the bottom you'll see that they're all running the free W3 Total Cache plugin. Throw any of those sites on a commodity host like (mt) or even GoDaddy, with the same plugin, and your users will have a nearly identical experience.

I don't doubt that they're offering a good service, but you can achieve this on your own with the a cache plugin and a reliable hosting service. In todays modern VPS world, $49 and $99 (WP Engine pricepoints) will get you a pretty beefy VPS.

This post is indeed a bit vague on the details, and does feel like a post piece. A disclaimer should have been at the bottom :P

Also, I am curious to know if they're using mod_php with Apache, or FastCGI. The latter of which is more secure. At that point, however, I wonder why they just wouldn't use Nginx + FastCGI for everything and eliminate Apache altogether.

At the opposite side of the spectrum, it does look like they have a few other nice features like being able to snapshot your site and modify things in a staging area, separate from visitors. They don't go into too much detail regarding their backup system, but anything is better than nothing. Interesting to see a new service like this in the WP arena.

Hopefully WP is rewritten in fully OOP PHP5 so that developers can be happier, with some elite caching at the core. For now, it's totally burned me out.