More than once, a Windows user has walked by my Mac, seen my version of Skype, and said something to the effect of "Wow, this looks so much better than the horrible mess we have on Windows!" It seems Skype has noticed that there is a discrepancy in quality between the two versions, and has decided to make the two versions more similar. Unfortunately, instead of making the Windows version of Skype better, they've decided to fix the discrepancy by making the Mac version of Skype more like the Windows version.
Skype 5 embraced a single window model; one unified window handles placing calls, text chats, video chats, the contacts list, and more. It's awful. (On the plus side, it was the first version of Skype to support video conferences; you could video chat with up to three other people at once—though Skype charges a fee for the service.) You'll be hard-pressed to find a Mac user willing to defend Skype 5, but it's easy to find one who will criticize it: Call them with Skype.
So perhaps the best news for Mac Skype users is that there's almost assuredly no way Microsoft's ownership of the software could make things any worse. And perhaps the second-best news is that, should Microsoft find a way to make things worse on the Mac side, we'll still have plenty of options.
iChat, of course, comes free with every Mac, supports free video conferencing (for up to four participants, with audio chat supporting ten participants), and sports an interface that shows some familiarity with Apple's interface guidelines. Now there's also FaceTime, which makes it easy to call not just other Macs, but iPhones and iPads, too. If you need to video chat with Windows users, iChat can work, if those users run AOL Instant Messenger, though in my own experience it's often a challenge. FaceTime remains Mac/iOS only.
Luckily, there's also Google. Google supports cross-platform one-on-one video chatting through Gmail—although it requires Flash. Google can also recreate Skype's ability to place calls to (and receive them from) actual phone numbers. Throughout 2011 (and potentially beyond), Google's Gmail integration with its Google Voice service is free for calls to the US and Canada.
As referenced earlier, however, the Skype iOS client is actually quite good. Microsoft's iPhone apps are generally good as well, so iPhone users can feel optimistic that the app won't take a sharp turn for the worse. Should that happen, FaceTime is the best major alternative.