For those of you unfamiliar with it, Twitter is a service that allows you to send out ?Tweets? (140 character long messages) to their network. Each user has the ability to follow other users, send direct messages to other users, and reply to Tweets from other users. The service is fairly popular in the tech industry, allowing for people to keep up to date in a micro-blogging fashion. The Twitterrific iPhone app is a 3rd party app for Twitter that allows you to do quite a bit directly from your iPhone.
There are two version of the Twitterrific: the free version and the $9.99 version. This is one of the few apps to be released that actually has a free version that does all the same features as the full version. The only thing that you have to deal with is an ad placed at the top of your main Tweets page. This is definitely something that you can put up with to save yourself some dough.
Just like the Mac widget by the same name, Twitterrific doesn't really limit you on what you can do with Twitter other than the imposed limit of 70 requests per hour. This is something that all 3rd party Twitter apps deal with, regardless of whether they're on the iPhone or not. The added functionality of being able to transmit your location and post a picture directly from your phone do make it a useful app however.
But where Twitterrific fails isn't it's fault. Although the interface is overwhelming at times and often difficult to navigate, it doesn't add anything new for Twittering on the iPhone. The fact that you need to keep the app open to receive new Tweets makes everything kind of useless when you consider that you can easily set your Tweets to be received as SMS if you truly want to stay active. Twitter isn't something you use casually, it's something that people who are into it commit their time to it, so there really is no reason to use a Twitter app for the iPhone at all.