#1 I wouldn't use this app for all my bitcoining. An offline "savings wallet" is important to have. Here's a nice sticky with info and useful links: https://bitcointalksearch.org/topic/howto-create-a-100-secure-wallet-17240
Getting the App
On your idevice open the App Store, search for "Blockchain". Download the app, it's free. Alternatively you can find some info on it here: http://itunes.apple.com/us/app/blockchain/id493253309
It doesn't require you to enter any sensitive info. A strong password is always best.
Sending and Receiving Coins
In the app there are up and down arrows at the bottom of the screen. Choose the down arrow for bitcoin payments to you. If you tap on your address a small menu comes up, select "Request Payment". From there you get a QR code like this one:
These are becoming a popular way to get info to a mobile device through the camera. Soon nearly all mobile devices will have cameras and a connection to the net. Sending coins is just as easy as long as you can scan a QR code. Tap the up arrow and select "Scan From QR". Your camera should activate letting you point it at the code you'd like to scan. Otherwise carefully entering the address (and double checking it!) or a copy/paste are the only options.
Conclusion
With the ease and speed of the setup of this app plus the no fuss QR code feature I think it's a real winner. It seems like a good way to take a few coins with you when you are on the go or to receive coins without hassle. The sleek user interface, colorful screens and portable nature of it gives a new face to bitcoin too. This could easily be a conversation starter and a tool to help get bitcoins into the lives of everyday people.