In the past, if you wanted to try out a developer's kernel and then decided you liked your original ROM's kernel better a few days later, you had to either locate and flash the original ROM's kernel, restore a Nandroid backup and potentially lose a couple of days worth of data, or reinstall the whole ROM from scratch.
But there's an easier way that just involves having the ROM you originally flashed handy. We covered Flash Image GUI before, but there have been some great enhancements to it in the last few versions. One of these is to extract a kernel image out of a ROM and flash it separately.
Say you've flashed a new kernel on a ROM that was working perfectly fine. If the new kernel doesn't sit so well with your particular setup, just open Flash Image GUI and select the original ROM you installed.
While no substitute for a Nandroid backup in the event of kernel disaster, you can potentially save 20 or so minutes waiting for your backup to restore the original kernel. It's incredibly useful if you're just wanting to dip your toes in another kernel's waters without the possibility of losing digits.
Flash Image GUI is still available for free from the developer's page, or you can purchase it for $0.99 and buy him a PBR at the links below.