I'm running CM12.1 on my One at the moment and it's leaps and bounds better than the stock CM11S ROM that comes with the device.
1. Are they easy to brick?
2. Is there a decent guide for updating?
1. Not that easy to completely brick. If you try to flash the wrong recovery or boot, it will fail to start even in recovery mode (bricked?). I've done that to mine, but I was still able to boot it up while tethered (I have two phones and I inadvertently used the image for my other phone on my One Plus).
2. Loads of guides on their forums or google in general. Also, Nexus 4 or 5 guides are the same procedure, just replacing with the relevant images and ROMs (the OnePlus One's codename is Bacon, in case you're looking at what roms to download).