Firefox OS might be a viable options for low end phones, and For high end phones Ubuntu OS might be worth it as well.
The nice thing about something that works well on low end phones, it works awesome on high end phones :) Although its not necessarily something you get for free, since we havent optimised for higher end devices you can get to a point where you have too many active apps open and chew battery, there are probably other places it can improve, but it does perform nicely on higher end devices.
In any case, an open OS is a necessity and I'm glad that I didn't have to choose between iOS, Android, and WP for my first smartphone. After some major bugs have been addressed, I look forward to being able to buy a high end device on which I can install Firefox OS. I sincerely hope Mozilla continues to give the project a lot of attention and doesn't relegate it to Thunderbird status.