Twiter for Mac is the only app that allows me to monitor all my Twitter accounts in one window. Tweetbot is nice, but its entire side bar is one account (at least the last time I used it) whereas Twitter for Mac (formally known as Tweetie) is the only Mac OS app that lists all your accounts on the sidebar with notifications of new tweets, mentions, and DMs. So while many think Twitter for Mac's update has only caught up with Tweetbot, I beg to differ on this point alone. The rest of the updates were sorely needed however. The quote tweet function alone is worth it.
My only gripe with Twitter for Mac so far is that its window is bigger now. I liked having a thinner window, especially on a 13" MBP. On an iMac it's not so bad, but I like the really thin screen real estate. On the MBP I can only see 4 of my 5 accounts. Granted, the 5th account hasn't been actually used in years, and I've considered deleting it, but it was still visible in the previous version.