I've been following @exyle here on Steemit for some time and was excited to hear that his group @blockbrothers were releasing an app for IOS. I downloaded it the first day it came out and was not disappointed.
Firstly, it's incredibly simple and easy to use, you don't have to input your private keys or password. The only data you need to give the app is your username, meaning it's completely secure and impossible for you to be hacked. They actually initially had the app rejected by Apple because it was too simple and they thought @blockbrothers had made some kind of mistake by not including a log in function. I guess Apple just aren't up to speed with the power of the blockchain yet!
The app sends notifications when somebody replies to you which is useful because the Steemit notification system appears to have been broken for a while now. You can also read them while away from your computer, great if you're out somewhere and have a couple of minutes to kill. However, the most useful function I've found is that it notifies you when people upvote you, and what percentage of their voting power they used. Also, the app tells when somebody starts or stops following you. This is information which is not easily available on Steemit, and in opinion, is what makes the app so useful. Anything that presents new information in a more accessible way helps to deepen the user experience and I have definitely found that having this app has kept me more engaged with Steemit.
At the moment the app is only available for IOS but I know @blockbrothers are in talks with Android developers. It would be great to see it available for everyone.