With a separate /home partition it's easy. I installed Mint 17 (previously Mint 15) the other day (backed up /home anyway, just in case) and went with the custom install option (i.e choose your own partitions).
During the install I re-formatted my root (/) partition for the new version and selected my existing /home partition as the new /home mount point. When asked to create a user for the new install I entered my old username and password and my (ecryptfs encrypted) home folder was recognised and decrypted.
Finally I just had to re-install a few apps, my settings stored in ~/ were automatically recognised of course. Total time, <1hr.