To address Samuel's points above....
I've created a project on CodePlex that is called IsWiX that addresses the democratization issue. You use it with WiX to create Merge Modules and then consume the Merge Modules with InstallShield to get the best of both worlds. This allows a setup guy to use InstallShield and dozens of my developers to use IsWiX/WiX. The XML can still be marked up with additional metadata so we aren't restricted in what the modules can describe.
InstallShield has a stand alone build engine that integrates with MSBuild/TFS and provides an automation interface. There is no advantage for WiX here.
InstallShield is a text file also. It's an uglier DTD format but IsWiX solves that problem by abstracting the frequently changing portions from the rarely changing portion of the installer.
I highly encourage the use of DTF with InstallShield. After all a Type 1 Exported Function is the same to any MSI based tool.
InstallShield has a direct editor which shows you the underlying tables. This is actually closer to the metal then WiX which uses an XSD based DSL to output the metal.
All in all, there are really good things about WiX AND InstallShield and I use them together to create extremely complex installers.
PS- IsWiX put a lot of thought into hashing and sorting to solve branch merge problems. ( We use Base Clearcase on dozens of branches so this was very important to us. )
