Rather than keep interrupting myself throughout the review to talk about using Windows with OS X Server’s services, I thought I’d lump it all together at the end for convenience’s sake. There's not much to say, so I'll be brief.
Open Directory: Past versions of OS X Server were able to serve as Primary Domain Controllers (PDCs) for Windows computers, which is to say they could provide Windows systems with authentication and permissions for users and groups, even though they couldn't manage many Windows settings. Lion's new SMB implementation removes that functionality entirely, which is going to hurt for people who rely on it. Interoperability with Active Directory domains remains as robust as ever, but it just got that much harder to get by with just OS X Server if you have many Windows clients to speak of.Address Book, iCal, iChat, and Mail: All of these services use open protocols (or, at least, protocols that are supported by several non-Apple programs), so you can access them from many different products across many different platforms: POP and IMAP for Mail, CardDAV for Address Book, Jabber for iChat, and CalDAV for iCal. You may not get quite as polished an experience as with the built-in Apple tools, but you should still be able to interface with your OS X-using colleagues (and, of course, the services that offer web clients will render fine on PC browsers).
File Sharing: Lion's new SMB doesn't affect file sharing with Windows XP, Vista, and 7 clients - it all works as intended.
VPN: Properly configured Windows computers should be able to make full use of OS X Server’s VPN service, but check out this Apple support document for some caveats and configuration details.
Web and Wiki: Naturally, as long as you have a Web browser and appropriate permissions, you can access and edit Web and Wiki pages from Windows just as well as any OS X user. Note that you may have the best experience using Safari, but I didn’t have any problems using Chrome or Firefox in my testing.
Other Apple-tailored services - NetBoot, Podcast, Xgrid, Time Machine, Software Update, and others - won't do anything for your Windows clients. If you’ve got a mostly Mac network with a few Windows users, or if you intend to use OS X Server mostly to manage Macs and Windows servers to manage Windows, then OS X Server should work well for you; if you just have Windows clients, though, or if your Mac-to-Windows ratio is high enough, the removal of PDC functionality makes it hard to get by with just an OS X server.
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