Im a new user of this app, and I love it. It has been updated so that users have access to all of the Grammar Girl podcasts. It has a search bar to help you find the right podcast for your grammar concerns. You can also e-mail Grammar Girl within the app, see her Twitter feed and her Facebook page, as well as get a glimpse into some extras thrown into the app as free gifts!
I regret buying the Grammar App, which is another app that recently came out. It cost me a $1.99 and its basically a 1-dimensional Power Point/flash card app that teaches very basic grammar (no punctuation lessons included). Its more for ESL and grade school students than adults who want to fine-tune their grammar. The Grammar Girl app is the app you want instead!