Hey, I was browsing around and found that Microsoft released the redistributable version of Windows Installer 3.0 a couple of days ago. Check out http://support.microsoft.com/?kbid=884016 for info about new features and some of the key bug fixes. Most of the new features are related to patching (patch sequencing, patch uninstall, better patch install performance and management). I remember the first feature that I noticed when I was trying out prerelease builds though - running msiexec /? will actually display the command line parameters now!! So I don't have to dig into the help docs each time I forget what the exact parameter name is for something I want to do via command line.
Note that Windows Installer 3.0 ships as part of Windows XP SP2 also, so it has been out in the field for a little while, but the redistributable package brings 3.0 to other platforms as well. There are a couple of pretty big caveats though - Win9x is not supported (meaning Windows 95, 98, 98 SE, and Me). Also, you have to have Windows Installer 2.0 already installed before you can install the 3.0 redist package. That is why you see the published system requirements of Windows 2000 SP3 or higher (since SP3 contains Windows Installer 2.0).
Anyway, I encourage you all to try out 3.0 and let me (and others at Microsoft) know what your experiences are - what is good, what is still broken, what features would you like to see in future versions?