Messages works just fine as a client for the standard IM networks, too--AIM, Yahoo, and Jabber (and, by extension, Google Talk). As noted, it also supports SMS messaging; you just type the phone number in that To field. (You must have an AIM account enabled to do so.) In fact, one of the nice things is being able to mix-and-match chats with people on all those different systems at once. It still supports audio and video chats and screen-sharing on those other services.
There is, however, an odd split in the way Messages manages video chats. If you want to vid-chat with someone, you click on the camera icon in the upper right. That produces a drop-down showing that person's various addresses. Those associated with iMessage come first. If you select one of those, Messages will switch you over to the FaceTime app. If you select an address on AIM or another standard IM network, you'll stay in Messages app for that session. You can switch from a text chat to a video conversation in midstream, just by clicking that camera icon.
Messages obviously makes Apple's own iMessage system the first among equals: Messages can use AIM and those other services if you insist. But everywhere you turn, iMessage is the favored choice. In theory, that may not be such a bad thing: When the iMessage system works, you can do more with it than you can with those other services. But that's only the theory.
In practice--during the beta-testing phase and in our brief time with the shipping code--Messages has been hit-or-miss at best. Sometimes chats appear on one of your devices but not another. Transcripts that are supposed to follow you from device to device don't always do so. As we do more testing, we'll let you know how to troubleshoot these and other problems that may crop up.