When Apple introduced ActiveSync and Push email support for iPhone email, there were two reactions: 1) "Yay!" and 2) "What's this going to mean for battery life?" Any sort of "push" technology will reduce the battery life, but it is a trade-off many people are willing to make, especially when it comes to email.
Mail Notifier is one of several applications in the App Store designed to avoid the battery drain of keeping an active connection to your mail server. I've been using it for a couple of months and it has worked very well with my Gmail (actually, "Google Apps") account. In fact, there have been times when I was sitting at my computer and my iPhone told me I had mail even before I saw it in Mailplane. Does it actually save on battery life? That's a difficult question to answer without doing tests in a controlled environment, but anecdotally and unscientifically I would say that it does seem to have improved my battery life.
One of the nice features that Mail Notifier offers is the ability to set a "silent period" when you won't be disturbed by alerts. This is something that I wish Apple would provide globally, i.e. "Disable all push notifications between 11pm-8am" or each push application would implement individually, e.g. "If it is my move in Words With Friends during the day, play a sound and display an alert, but if it's my move between 11pm and 8am, just display an alert, but keep quiet. I might be sleeping."
Mail Notifier is also said to work with free Hotmail accounts and AOL, although I did not test either of those. If you use webmail instead of Mail, you can setup Mail Notifier to open a web page instead of the Mail app. You can also setup different sounds for different accounts. That is a handy function if you have more than one mail account, or if you have a "multiple iPhone household" and want to avoid the "Which one of us just got an email?" confusion.
Overall, the app has worked very well, and as advertised. However, there is some "small print" to attend to. I was surprised to see a large number of 1-star reviews on the Mail Notifier iTunes/App Store page. Then, I noticed a list of "top in-app purchases" for Mail Notifier. As I scrolled down to read the comments, I found that most of the 1-star reviews seemed to be about the fact that Mail Notifier is actually a subscription service, rather than just a one-time purchase. This came as a surprise to me. I was provided with a review code (see note below) that must have included at least 1 year of service, since I have not seen any renewal notices in the app. I did some looking around to see just how obvious this would be to a new user who happened to be browsing the App Store.
My conclusion is that it really depends on whether you came in through iTunes on a Mac or Windows computer, or whether you used the App Store on an iPhone or iPod touch.
Read on to see more.