Once the package is installed, you effectively have Windows running as an application under Linux (which means, importantly, that you don't escape the need to buy a licence for Windows). So to all intents and purposes you have a Windows machine, and you can install and run whatever you wish on it, just as you would if the PC were running Windows natively. Because the Windows world occupies a part of the disk underneath the logged-in user's home directory, it's straightforward to copy files between the two operating systems. Since Win4Lin can interface Windows to the network via Linux's network drivers you have the same access to the network as you'd have under Linux - though you have to be careful which option you choose, because there are two alternatives and the simpler one to configure doesn't allow you to use Network Neighborhood [sic] or connect to an Exchange server. It's hard to fault Win4Lin. It's fast, and because you have a complete installation of Windows running inside your Linux machine, it works just like it would if it were sitting directly on the hardware. So we were able to run Windows Update, install and use Microsoft Office, and generally feel like we had a Windows computer, all in a window inside the Linux world. The only niggle in our minds is whether you'd bother with Win4Lin when you can have a dual-boot PC without the need to spend your $90 or whatever. But given what a faff it can be to switch between worlds each time you need to on a dual-boot machine, we think that actually there is value for those whose lives exist in both a Linux world and a Microsoft one.
When considering this type of application, bear in mind that for an office-based setup, the Terminal Server edition may be easier to manage than a load of desktops, while for portable users the Workstation edition is more logical.