Every Guitarist should own one!
I bought one for my grandson who is just taking up guitar and decided to buy two and keep one for myself. I figured it would be a good idea to learn and teach him how to use it. It turns out this thing is so easy and intuitive that he figured it out by himself. I am so glad I did, this thing is fun. When you first plug it into your iDevice you are prompted to install the App. Just do it. I use it with an iPad2 and iPhone 4S. It works much better on the iPad, the larger screen making effects editing much easier. With the Mobile In and a decent set of headphones you get a fantastic sounding guitar rig. And there is a built in feature to jam along with your Itunes music. The effects, amp and cabinet sims are all great, especially the compression and delay. I have also tried using this as an effects rack, plugged into a Fender Deluxe Reverb. It works really well, with the exception that it is a bit noisy. For this you will need to buy two new cables. Only one is provided with the unit and it is too short when playing live, because if you pull on it the device to easily disconnects from the iPad. Line 6 has nailed it with this device, mostly tanks to the fantastic app you get free after purchase.