GeorgeSomersTweetdeck has some nice features but a bit of a learning curve. I like the groups feature that you mention is nice and a great way to filter the "noise". I also like the server sync feature for using the client on multiple computers. The iphone app, however, constantly crashes even with only 4 columns. I have had to forego the use of mentions and @replies. On the desktop, I love has users with multiple twitter accounts can posts to multiple accounts AT ONCE. It is my current favorite on the desktop but I'm sticking with Tweetie on the iphone. Maybe I'll post my on boo on the topic soon.
