When Cerulean Studio made an alpha version of its Trillian software available for the Mac in early 2010, I wasn't very impressed. There were no buddy icons, the product didn't resemble the images released for it, there was no group chat support, a good many of the options didn't work, therer were no chat logs and more. At the time, I recommended sticking with Adium or Pidgin.
I'm very pleased to say that nearly two years later, Trillian has developed into a solid text-based instant messaging client for the Mac.
Trillian is available as a free download in the Mac App Store now, and it has gone through a vast number of improvements. While there's still not as many features as Adium, it's a solid client. Like Adium, it has integrated Facebook and Twitter, and I actually like these features better on Trillian than Adium.
It also syncs with the free Trillian for iPhone, which means you can start a conversation on your Mac and pick up where you left off on the iPhone if you need to run somewhere. The sync is the standout feature for Trillian, and for those of us who need to maintain IM sync on the go, that might be the feature that gets you to use this.
You still have to register for a Trillian account if you don't have one, but since you gain syncing ability for chats and settings, it's not as nitpicky of an issue as it was for me last year.
Some of the cons I found last year still exist. There is still no group chat ability, which prevents Trillian from adding IRC. It's light on customization, for that you'll need Adium. There's no audio/video/Skype integration, but that's something I've lived with regardless because I use Adium. Not all the buddy icons come through OK, the most success seems to be the ones associated with AIM and Yahoo accounts. You can log chats now, which that alone makes it worth considering for me as a Mac client. I like the way the chat logs are handled. Right click on each user, and you can access the history, including a calendar for easy access to certain dates.
Trillian Pro is available for $12 a year, which allows chat logs to be stored in the cloud and no ads, though there are currently no ads in the Mac version anyhow.
If you need a solid text-based IM client with updates handled through the Mac App Store and syncing, Trillian is a good way to go. If you want more features and customization, stick with Adium. If you want audio, video or Skype, you'll be stuck juggling multiple clients regardless.