The only problem with RFID fobs is that they are VERY POOR for security. They're great for inventory and other tracking applications but not well suited where security is an issue. It's extremely easy to 'copy' somone's RFID fob with hardware readily available. And someone can copy your fob by just walking past you. Same issues actually exist for the new keyless ignition systems in cars using start buttons, etc. It's extremely easy for someone to steal your car by copying your fob.

The company I work for produces library self check systems and we work with RFID all the time.