Wont ruin the warranty, jailbreaking is totally reversible, as long as you sync it before you jailbreak which is normally done anyway. If there are any problems during the jailbreak, which is unlikely. The phone can ALWAYS be put back to its factory settings, and from there, you can sync it to your backup file thats created.
Be warned that it does slow the phone down aswell, but the benefits such as the free apps are totally worth it, so are all the tweaks that you can do.