So, what is ReactOS exactly?
I could post more screenshots, but you'll say it looks just like Windows 2000, with some weird added icons.
What ReactOS actually is, is a completely win32 binary compatible OS. So in short, you can use it instead of Windows with all your programs working, while that's impossible with linux or MacOS without an emulation layer (or virtual machine).
What does it mean for the OS world? Well, not much, for now, since it's still in alpha stage and some important things do not work well. Almost all new computers already come with some version of the latest windows installed, so you don't really need an alternative.
But when it'll be compatible enough that there will be virtually no difference when running the two, I'd safely bet that ReactOS will takeoff and the development of patches / security holes will be as fast and efficient as any community-based linux distro. That means it'll be faster and safer than its current windows couanterpart.
And at the end of the day, a stable ReactOS will be the OS of choice for POS, terminals, internet cafes or any other usage that wants to avoid an expensive license.
And who knows, maybe in some years we'll all be wondering why people still pay for Windows...
If you want to try reactos, download the version preloaded with QEMU, a free virtual machine. In 10 minutes time you'll be running it on a window on your Windows OS without installing or modifying anything.