Should be 100% safe - nothing on disk is changed besides the root binaries, so if something goes wrong, simply rebooting the device should fix up any and all issues. Let me know if you find any bugs!
Here is the list so far of known compatible and incompatible devices:
Samsung Galaxy S (All variants)
Galaxy 3 I5800
Desire (requires nand unlock)
Desire HD (requires nand unlock)
Evo (requires nand unlock)
G2 (requires nand unlock)
Archos 70 (unknown)
LG Optimus One (unknown)
myTouch 3G (unknown)
V1.0.0 : Initial Release
V1.1.0 : Will now open the application settings page if USB debugging is not enabled.
V1.2.0 : Unroot support, fix for devices with small max file size such as I5700, binaries in xbin rather than bin
This application is available directly from the market. Search for 'z4root'
按ROOT後會自動重啟, 即可ROOT（需要開啟USB debugging）