HootSuite also has some nice extras, such as a "Send Later" button that lets you schedule your tweet rather than just send it immediately. HootSuite also makes it very easy to get information about users by clicking on their names; you get an easy-to-read pop-up that offers info such as their follower/following and their description. This is sleeker than a similar feature offered by TweetDeck, which creates a new column for that data.
If you're looking to aggressively promote yourself or your company, there is also a direct link to Ping.fm, a service that allows users to send out their tweets to multiple social networking sites.
Finally, HootSuite uses its own URL shortener, Ow.ly. Like the third-party Bit.ly URL shortener service, it enables users to not only shrink long URLs, but also check how many people have clicked on that URL as a result of their tweet.
What needs to be fixed? To begin with, the folks behind HootSuite should learn that good word of mouth can't be forced -- to try a beta of the 2.0 version, I had to send a tweet supporting the product. If I hadn't been reviewing the service, I would have been strongly tempted to decline the honor.
And there are features that still need tweaking. For example, if you want to create a group, you have to type in at least one username, which can be a problem -- usernames can be strange enough that it's difficult to remember them exactly. Once you've created the group, you can drag and drop users from another column to your group column to add them to the group, but that still means a search for all the users you want to include. Mixero, which provides you with a list of your users, makes creating group much easier.
Final verdict: For a Web-based Twitter client, HootSuite offers some nice features and a few that other free Twitter clients don't, such as a convenient way to get a hit count through the Ow.ly feature and tabbed pages. If you prefer a Web-based interface and are using Twitter as a way to promote a brand or some other commercial venture, this should be on your short list.
While most Twitter clients seem to be standardizing with an interface that uses columns to help users follow multiple feeds, Mixero is experimenting with something a bit different: a set of panels that let you quickly switch among your feeds and searches. This beta desktop client may not work for everyone, but with a bit more development from the Mixero team, it could be something really outstanding.