When you first run WPI, you can set things up roughly one of two ways. The "easy" way is to select Applications, pick the app(s) you want installed, and let the installer do all the heavy lifting. Any components needed for the installation that are not already installed will be auto-selected and installed. The installer also asks pertinent questions about the app(s) you're setting up: default usernames and passwords, database connections, and so on. It's all very professionally presented and deployed.
The slickest part of the Web Platform Installer isn't the Installer itself, though that's nice on its own. It's the WebMatrix control panel application, which provides a management interface for websites deployed through WPI. From this interface you can perform basic administration on the website (start/stop/restart), jump to the Web-based configuration pages for any installed applications, see a log of all requests fulfilled by the site (with a handy search function), examine databases and files associated with the site, and even run SEO/performance reports on a given site.
Another of WPI's big strengths is that it works as a staging server, from which content can be automatically deployed to a production site with a couple of clicks. This includes everything: the files on the server, data in the database, everything. Unfortunately it doesn't let you do the same in reverse; you can't slurp up a remotely published site and edit it locally unless that site was deployed from WPI in the first place.
Another issue I ran into: When an app is deployed in WPI, it's deployed in a stack that has its own port number. If you want to change this port number, you'll need to change it by hand in WebMatrix and the app's own internal settings. Also, the default hostname for the app is localhost, so if you want to make the app accessible by anything other than the local system you'll need to change the hostname (again, in both WebMatrix and the app itself), and open a port in the system's firewall for good measure.
Recommended for: Those developing on IIS on Windows Server, but using open source stack components as well.
Microsoft's Web Platform Installer lets you pick applications and even whole frameworks, by Microsoft and others too, to install into your stack.
WampServer 2.2 A stack similar to XAMPP in construction, but with fewer components in the stack, is WampServer from AlterWay. This stack seems to be aimed most directly at the PHP programmer, since it ships with a set of PHP debugging and tracing tools. Debugging has long been one of PHP's Achilles' heels, so it's handy to have a stack shipped with components that address that issue.