With an encrypted filesystem (File Vault), you have to re-boot into an ultra-secure mode. This is almost analogous to a Chromebook and I like it. The new guest mode only has one app running and that is Safari.
Once in guest mode, the user has no other access. He/she cannot access any applications nor can they browse the filesystem.
For comparison, here is the guest mode on my iMac also running Yosemite without File Vault. The original guest mode has desktop, application and file system access.
So if you are running a full file-vault, the guest mode will be a complete surprise. I can see some people not liking it; preferring it to the original mode. I personally like it as it appears to be more isolated and there is little to no chance a user can see anything on my drive as it is intended.