I have been looking to switch to OAuth for my Twitter integration code and now that there is a deadline in less than 7 weeks (see countdown link) it is even more important
I've used bengottlieb's Twitter-OAuth without issues.
Build+Analyse finds only one issue (in SA_OAuthTwitterController.m) and that's just a variable that has a value stored during initialization that is never read. I've not seen any leaks in my use of it. It was simple to implement and so far (two apps in the store using it, a third in development) no problems at all.
A new single-file solution is TDOAuth: http://github.com/tweetdeck/TDOAuth.
It's well-tested in that it is used in the TweetDeck iOS clients.
I recently had to implement "post to Twitter".
What I found was that none of the various projects would work out-of-the-box. They're all close, but not quite there.
Eventually I settled on OAuthConsumer, Stig Brautaset's SBJSon, with some ideas more than code borrowed from Ben Gottlieb's Twitter-OAuth-iPhone for pin processing.
I probably would have looked at MGTwitterEngine, but I didn't need to hit that much of the API - just status updates.
I seem to recall having to hack OAuthConsumer to set a few parameters correctly - that was the biggest stumbling block.
Try this Framework out for size: https://github.com/materik/meauth-ios, works well for BitBucket's API and am in progress of testing it with more sites. Please give feedback and contribute.