Three things:
1. Even if I were interested in a Fire Phone, I'd wait to buy one until at least the second iteration. It's rare for anyone to get a new product line right on the first try -- let alone with as many new services as this.
In terms of first-gen phones, I'm more interested in the
OnePlus One. OnePlus has made the One absurdly difficult to buy for the mome, and the difficulty is intentional, but that's aces with this compulsively hesitant fussbudget. Let someone else beta-test the One and list the drawbacks they discover (whether it allows USB audio and file transfers, etc.). Since the One ships with CyanogenMod installed, I don't foresee user flexibility issues, but one never knows. Still, according to
iFixit, the battery is nigh impossible to replace.
2. I tend not to like walled gardens that can't be scaled easily and practically over time, which is why I've never owned an iPhone. If the FP had the potential to be rooted easily and was, if Amazon didn't make rooting impractical through mandatory upgrades and store/feature access, and if fundamental dependency on Amazon's ecosystem weren't burned inescapably into the device, then perhaps I'd buy the off-contract version if no other phone interested me.
3. Amazon tablets sometimes offer better audio than certain other competitive devices, but I have no idea whether the FP allows USB audio (via an OTG cable). If it does, and if its speakers and DAC chip prove especially good, then the FP could become an option for music lovers -- even more so with the various services offered by Amazon.
[Edit: Since the FP uses a Snapdragon 800 processor, the DAC chip is just as mediocre as every other U.S.-specific LTE smartphone's.
Most international versions of Samsung devices with the Exynos processor, like the S5, tend to use a Wolfson DAC and are clear winners in terms of sound but not data speed.]
As for cheaper carriers:
I prefer Ting over Wireless Republic because of the BTOD option, but both carriers are problematic for me because GSM is better for those of us in the U.S. who travel to other countries. Ultimately, Sprint should to switch to GSM and probably will, so I'd hate to find out that was happening right after buying an expensive CDMA device.