... Without me presenting in front of computer and log into my computer, they cannot get access to my data, right?
Right. Actually, it is not required to be physically present to log in and use your computer, but your login name and password are always required to access your data. This assumes that you have configured Sharing Preferences to allow this, and that whomever is attempting to log in already has access to your local area network, which requires either physical access to its infrastructure or your wireless network name and password. Those are several layers of protection to get through.
File Vault (a/k/a Vile Fault) mostly exists to make life inconvenient for you, should you encounter a problem with your computer.
A very determined person could extract your data, even if it is encrypted with FileVault, using some very expensive utilities that run on very expensive hardware, but physical posession of the computer would be required. This would require tools and abilities that far exceed the Genius Bar's.