A good reason to upgrade is that it brings important runtime performance improvements in PostSharp Laos thanks to better caching of calls to System.Reflection and thanks to a custom implementation of Delegate.DynamicInvoke (the default implementation provided by the .NET base library, relying on dynamic invocation through System.Reflection, is just shameful).
This version is side-to-side compatible with PostSharp 1.5. Pay attention to this breaking change: the version number of PostSharp libraries now does not contain the revision and build numbers anymore, so the new version is 1.0.0.0. When referencing PostSharp libraries from your projects, you need to enable the "Specific Version" property of the reference.
Bad news is that all plug-ins have to be recompiled so that they have the proper version number; good news is that this is the last time, since later maintenance releases won't change the version number any more.
(Note that the file version still contains the revision and build numbers, so you can always check what version you are running.)
As promised, there is no change in the license and this version will continue to be maintained until 1.5 becomes stable.
Happy PostSharping!