Surprisingly there are not a lot of options for Objective-C Twitter libraries. In fact there are exactly two options: MGTwitterEngine and Canary (which is actually a “full-fledged client”). The choice looks clear, use MGTwitterEngine or deal with XML directly (yuck!).
MGTwitterEngine does not implement the entire Twitter API but it does implement the most important things like updating status, retrieving your friend’s timeline, etc. For my current uses, that’s all I need.