This is just a small maintenance release that fixes a couple of annoying bugs:
a) The logout/session issue should now be resolved permanently.
b) The problem with errors in preview on new installs because of some bad logic in the default templates has been fixed.
Diff packages are available (they are very small) to allow you to quickly upgrade your 1.5.3 site to 1.5.4.
This is an optional upgrade. If you’re not experiencing problems with the logout/session issue, then staying at 1.5.3 for a while should be fine.