Twitter is a useful way to get more traffic, and best of all, it can be entirely free. The only downside is spam and services like truetwit, luckily however, there isn't really that many using them.
Horrible UI and Poor design
Twitter has a poor design, it takes to many resources due to heavy use of scripting, and the UI part of the design is just down right horrible. It toke BlueBoden a long time to figure out how to delete messages in the inbox, and that is because the delete link/button only becomes visible when you hover the message with the pointer.
As if that wasn't enough, you also have to confirm that you want to delete the message, preventing a smooth experience of the PM system. The main problem with the PM system on twitter, is that its abused by spammers, it would be better if you could choose only to accept PMs from certain lists.
The second problem is the UI, which could use some improvements. Basically get rid of the confirmation dialog, and the invisible delete control can be forgiven. This would allow quick deletion of spam messages.
Services like truetwit are just a pain, especially because they use the PM system on Twitter, which suffers from spam and issues with its UI. Most serious twitters likely rarely check their PMs for this reason.
Truetwit also uses recaptcha, without providing any alternative for people who don't want to support such projects, in this case to "digitise books". Its also pointless to use services such as these, as it limits your exposure. Each listing, no matter if the user is real or not, could potentially mean more followers to you.
You shouldn't abuse twitter, and just follow everyone, but following relevant people in your field may give you a traffic boost. Only follow people you find interesting, and those following you back. Non-backfollowers can safely be removed after a day or so, tools like justunfollow.com can be used to easily keep track of who's following you back.
Sadly we didn't find any free tools, so we chose to mention justunfollow of the many, whom recently also got a feature to show inactive people you follow. Don't go nuts now, some people can be inactive for months without really being inactive, so I'd say don't set this for anything less then 6+ months. And if you can, try to keep your in-actives regardless. Even old inactive accounts sometimes still have many visitors, some whom just follow users from follower/following lists.
Twitter and search engines
Google has been talking about making sites like Twitter and Facebook effect your rankings, so this is just another important area, where you can maintain some control over your rankings. Its still more or less unknown how these sites effect rankings, whether its directly, or just for the latest search results. Likely the latter though, considering some twitter users have obtained hundred of thousands "backfollowers".