So far features I really like include saved searches, universal @ responses in Twitter (every mention of you) and the ability to record Seesmic videos from Twhirl itself (I am using Seesmic more and more lately, it is a lot of fun and using it for video comments is a great change). One thing I love about Seesmic is Loic’s enthusiasm and the fact that he takes the time to reply to as many videos he receives (it works a lot like Twitter with replies and the ability to send videos directly to people). This video is a really good example of his enthusiasm. It is infectious!
Another great thing about Seesmic is that it is integrated into Disqus so if you use Disqus to manage your comments on your blog (I recommend giving it a try if you are not), you can enable Seesmic comments too (as well as Facebook Connect integration!).
Loic has published a step by step screencast showing some of the features you will find in the latest version of Twhirl Some of these features will be familiar to you if you have been using Twhirl and some will probably pique your interest that much more than Twhirl does already.