I'm willing to give Spartan a chance, honestly. The ModernIE initiative taken by Microsoft the past few years feels like Microsoft gave IE to a team of competent engineers and told to run with it. They dropped support for older versions of IE and upgraded the technology behind it.
Firefox is too slow. As a power user I notice millisecond differences in page render times. I may be an outlier but I think most people who spend a lot of time on browsers will notice even small differences in load times. I've long switched from Chrome as my main browser, disliking the direction Google has been going with several of their products.
I might be an outlier, but I'll give Spartan a chance to impress. [As a side note, it's amazing how crippling to your reputation bad technology decisions are. IE 6-9 gutted respect for Microsoft]