I am a recent convert to Twhirl. It is my new favorite Twitter client. I formally used Twitterific. Twitterific is a great client still in my opinion, but the ads were a bit distracting and I didn't want to pay the $15 dollars for the full version (Okay, so I may be a little cheap, but I can't see a reason to pay for something when there are free alternatives.) The only feature I see Twhirl missing is Growl. I asked @twhirl (on twitter) if supporting Growl was planned to be supported. As of now, its not and doesn't look like it is going to be anytime soon.

I believe the lack of support is a limitation of Adobe Air rather then Twhirl. Air tries to be multi compatible with different operating systems while Growl is a OSX only application. Hopefully some day Twhirl will offer support for Growl. Its not a ruiner for me personally, It has its own notification system that doesn't interfere with Growl. It would just be nice to have one notification system running since all other applications I use have support for Growl (where necessary).