As described by the producer, it should be 100% safe – nothing on disk is changed besides the root binaries.
This means that if something goes wrong, a simply rebooting should fix up any and all issues.
This application is available directly from the market if you search for “z4root” or you can get it here: here.
Disclaimer: rooting your phone will void your warranty and can brick your device, so use this tool at your own risk.
For me this tool worked like a charm, I have Samsung Galaxy S with Froyo (2.1).