A few days back, we got our hands on a build of Windows 10 with Spartan and even though Microsoft has not released a build with the browser, you can now try it out for yourself if you are willing to install a leaked build.
An older WIM file of Windows 10 build 10014 has leaked that has Spartan enabled and if you know where to look, you can download this file and try out the new browser for yourself. It is worth pointing out that this particular build is highly unstable and despite having Spartan, it's probably best to skip this leak.
First off, the build number is 10014, Microsoft released 10041 last week, which means this is an old build. The second reason is Microsoft is going to start pumping out builds at a much faster pace than before which means you shouldn't have to wait too long to try out Spartan in a stable environment.