I too will give Spartan a chance, and in likelihood will use it predominantly on my tablets and laptop (since I use IE 11 in those contexts today).
On the desktop, I presently remain a Mozilla partisan, sticking with Firefox through thick and thin. But I have acknowledged that in terms of pure page user interface and interaction speed on a Windows PC, nothing can compete with IE. Its use of GPU acceleration for 2D page rendering makes things like CSS transitions shockingly smooth and fast. The CSS transitions for gradients are particularly delicious; I still have an optional subtle animation in the background of my blog using the gradient transitions. I used to have more animation in my blog (before readers convinced me it was annoying), and IE made Firefox and Chrome look as if they were running on clunky hardware from 5 years ago. IE is the only browser that—even when rendering typical web sites—reminds me that my workstation is a 3770K with a high-spec nVidia GPU. In Firefox and Chrome, I can easily forget how powerful my PC is until I fire up something that kicks off some 3D.
I am hopeful that Spartan will continue to lead in that area of so much importance to me: perceived performance. If it improves its actual JavaScript performance and CSS capability versus IE 11, which I expect it will, I'll likely be quite impressed.