Just like working with my Twitter stream, I can easily reply to people (as you can see in the screenshot). I can lookup users and subscribe to them. I can see my own posts and also replies to me. Twhirl also has a very cool feature where you can easily see in the client who you are following and who is following you. (You can't do this for Twitter in Twhirl.)
For me this makes identi.ca infinitely easier to use. There's also a XMPP integration that allows for real-time receiving of identi.ca notices... which sort of turns Twhirl into almost an instant messaging program. I've not tried this yet, but the tutorial shows how easy it is to set up. Nice feature (and something you can't do with Twitter).
Separately from Twhirl, there have also been updates to other Twitter clients Posty and Spaz and a new IndentiFox client (a spinoff of TwitterFox). Additionally, a Twitterific user figured out how to hack it to work with identi.ca. Naturally there was a good amount of blogosphere coverage. Here are some worth reading:
- Read/Write Web: "Twhirl Adds Identi.ca, Becomes Killer App (Yes, Really)"
- The Inquisitr: "Identi.ca Latest: Indentifox, Posty, new features"
- sarahintampa: "Identi.ca Adds Features, Gets into Twhirl" (Good commentary on why people use identi.ca, too.)
- Louis Gray: "Posty's Single-Window Microblogging AIR App Adds Identi.ca Support"
- lmorchard: "Using Twitterific with identi.ca"
It's great to see... and let's see the support for identi.ca continue to grow! Good times...
P.S. If you are experimenting with identi.ca, feel free to follow me at identica.com/danyork
 I'm not sure I'm thrilled with the word "dents", but: a) it's getting common usage; and b) I don't have an alternative.