The core interface has also been given a makeover. Similar to Twitter's own clients, your notifications feed now includes both mentions and activity like follows and retweets. A new sidebar (which you can collapse) lets you jump directly to a list, saved search or direct message thread. And if you use Tweetbot like a Tweetdeck replacement with multiple columns, it should be easier to manage those columns.
The app is available for $10 through the Mac App Store, including for existing users. That may sound like quite the expense, but Tweetbot (along with Twitterrific) is frequently considered one of the better Twitter clients on any platform, and considerably more powerful than Twitter's existing Mac options. If you treat Twitter as a vital communication tool, it might be worth the expense.
And in case you're wondering: no, Twitter's since-delayed developer changes don't signal doom for Tweetbot. If they kick in (there's no guarantee they will), they would mainly prevent Tweetbot from displaying notifications for certain activities, including likes an mentions. That's annoying, but not fatal. Notifications for direct messages and follows would be delayed by "1-2 minutes," the company added. You'll still have a viable alternative if Twitter continues to discourage third-party developers -- it just won't be quite as viable as it is today.