Tweetdeck is convenient for me since I use Twitter, Facebook, and Foursquare. However I still use Twidroyd for tweets with mentions because it has lists my friends I follow as I type. Once Tweetdeck has that I probably won't be using Twidroyd much.
Touiteur, Tweetcaster, &Twidroyd- Doesn't show when someone is following you back or when someone "Native" Retweets you. Touiteur doesn't even show saved searches. Uninstalled.
Seesmic- Doesn't show you who is following you back. Uninstalled.
Twicca- has EVERY function imaginable EXCEPT,when you get "Native" Retweeted, you won't know WHO Retweeted it. (Just like Seesmic).
Tweetdeck- shows "Native" Retweets in your mentions, just like "Editable" Retweets (and shows WHO Retweeted u... unlike Twicca).When you go to someone's page, you can see if they follow u, u follow them, or BOTH. Plus u SWYPE through colums. U can save Lists, Searches, or Someone's else's Timelines or "Mentions" as a column. No Trends or AutoComplete tho...but I can live without that, versus having other features I need.
With that being said, I'm back & forth between Twicca & Tweetdeck
Touiteur, hands down, no question. Besides the best interface IMO, the widgets blow anybody else out of the water. The only downside for me is no FB integration but it's not that big of deal honestly.