I started using Adjix because I saw Guy Kawasaki using it, and assumed it had something to recommend it.
I do not use the advertising portion of Adjix (which I would consider intrusive). Rather, I use it merely for convenience and analytics.
At the time I started using it, Adjix was one of the few applications that tracked how many clicks you got on a shortened URL. Also, Adjix allows you to schedule tweets in advance, something I find convenient if I happen upon something on the Web, but have just tweeted 2 minutes ago -- I don't want to overwhelm followers with too...