I am a confirmed Twitter addict and find it a tremendous resource for keeping up with the world. Professional notices, suggestions, advice, personal info, what's up with friends and colleagues --- always a lot going on, always something interesting. I like the ease with which one can follow others -- or quit following them. The key to using Twitter is to follow interesting people, to post regularly about things beyond what you're eating
I use Friend Feed, a social networking aggregator, as a way to post to Twitter as well as to follow friends on 'Twitter and on a host of other Web 2.0 applications (flickr, facebook, delicious, etc.). Actual conversations are possible on FF.
Posting from my phone is easy and, in certain circumstances, it is worthwhile to be able to receive tweets via SMS (text) messages.
I use Twitpics to post pictures and have set my Flickr account to directly post selected pictures to Twitter.
There are a gazillion apps that work with Twitter -- more than it is possible to keep track of. If you have a special need for a certain kind of application to work with Twitter, chances are that someone has already produced it.
My Twitter username is rcksolie -- feel free to follow me (and I'll follow you in return) or look through the list of people I follow to find others to follow. Hope to see you tweeting soon!