I'm not going to buy it.
Buying a Twitter app for $19.99 is unbelievable.
You're forgetting that there's the official Twitter client and TweetDeck, both which are free.
At first I thought the price was outrageous, but $20 for an app I use everyday isn't too bad. Especially since Twitter for Mac isn't going to be updated anymore. I love the instant updating, which the regular Twitter app also had, but I especially love the ability to "blacklist" certain words/tags, because many of my friends livetweet along with TV shows, and it's nice to avoid spoilers if I'm not watching at the moment. It just runs great and looks great.
Nothing compares to Tweetbot for Mac though! And although the official one is free, the development on it has stopped which means there won't be support for retina displays or any new features that Twitter rolls out.
$20 for a good piece of software is nothing! $20 to get the best experience for Twitter is something that someone has to base their twitter usage on...
Trust me, once you either forget to back it up, or it stops functioning, period, because of Twitter API changes, in a few months or so, you won't feel the same. More like, "Why did I buy $20 software that only lasted me 6 months?"