The ASUS Zenfone 2 has proven a popular device in its first few weeks on the market. It's reasonably priced and has good specs. As David pointed out in our review, the software is its biggest issue, so you know there are tons of users anxious to mod the crap out of it. Good news on that front—there's now a bootloader unlock method, and it's not terribly messy.
According to reports on XDA, you can unlock the bootloader all on your own if you're running the latest 2.19.40 firmware. This version hasn't rolled out to all devices yet, so that's the main caveat here. You'll also need root, which isn't hard on the Zenfone. Then you should just be able to enter the following ADB commands to unlock.
getprop ro.isn > /factory/asuskey
When you reboot, the boot screen should be white instead of black, indicating the bootloader is unlocked. Keep in mind this process is not reversible (at least right now), and you can probably break things if you're not careful. Use caution. At any rate, now that there's a bootloader unlock, developers can start getting custom recoveries and ROMs running on the Zenfone 2.