I've spent part of the day witih TweetDeck after weeks with Spaz, and have come away with some comparative notes.
- Simple tabbed single-column layout.
- Works well with the keyboard, allows you to scroll up and down, lightly greys items you've previously looked at.
- Scrolls pretty smoothly.
- Single or multi-column layout.
- Multi-column layout looks a little lame when most of the columns are empty.
- It's nice to be able to separate replies from the rest of your incoming messages.
- The ability to group people or have a few searches up at once seems handy, although I didn't use either, and I'm not sure how often I would.
- Feels jumpy when scrolling, easy to lose your place.
- Not well-suited to keyboard scrolling and difficult to know which items you've already read.
I've also played briefly with twhirl, but so briefly that I don't really feel qualified to comment. I found the skin to be more noisy and cluttered, so I quickly went back to Spaz.