What does it do? Seesmic is indeed a bit like TwitterDeck on steroids. Like TwitterDeck, it organizes your Twitter and Facebook feeds into columns. You can create new columns by starting a search or by following a single user or group of users. Or, if you want, you can just follow your Facebook and Twitter feeds in a single column (and it can handle more than one Twitter account).
In fact, rather than automatically putting your Facebook and Twitter feeds in two separate columns, it starts with a Home column in which both are combined. If that bothers you (it bothered me because it results in duplicate messages from people who send to both feeds) you can either choose to look at only one of the two feeds (by clicking on a menu to the left of the columns) or break one of them off into a separate column. You can easily send a status message to Twitter, Facebook or both, and Seesmic lets you add an image to your tweets.
What's cool about it? Seesmic is incredibly flexible; you can make it do almost anything you want it to -- and you can do it on a moment's notice. Say you've searched on a term and you're now curious about one of the tweeters -- just click on the name and you immediately have a new column with all of that person's recent tweets. If you're tired of that search, an icon at the top of the column will close it up or delete it.
Seesmic works on a system of "fixed" and "detached" columns. You can have a single fixed column that shows all your Twitter and Facebook accounts; you can then generate a series of detached columns that follow specific accounts (for example, just your Facebook account), specific users or specific searches. You can have nothing but detached columns -- or you can have a single fixed column.
If a column shows a specific Twitter account, a series of icons on the bottom let you filter it further into replies, direct messages (private tweets directed to you), etc. In any column, you can easily reply or retweet a message by simply clicking on one of the icons that are "hidden" within the user's picture (very much like TweetDeck).
Seesmic is one of the most Facebook-friendly Twitter clients I've come across. Each of your friends' status updates from your Facebook account has an icon that, when clicked, shows you the details of the message and lets you comment on it right from Seesmic.
And if you're not near your own computer, you can access Seesmic's Web-based interface, which is a simplified version of its desktop application. An iPhone version is in the works.