I agree with HTH, as long as the hardware is up to it, it should be great. I use my Micro BR all the time but the amp models are poor, If the iphone ends up better then it will be great.
You could look at it like you need an iphone, but if you already have one anyway then it's a lot cheaper than getting something else.
It's not though, is the thing. It'll still be around the cost of a vox amplug, possibly more. Ok it might be more versatile in that it has models claiming to sound like a variety of amps and effects but the quality won't be any good, otherwise it would cost a lot more. Plus the iphone hardware wouldn't be up to producing anything half decent anyway.
If I'm wrong and it's a £35 Axe-fx without the box then great, we can all buy one and someone will make some software enabling you to use the thing on a PC and we'll all live happily ever after.
If I'm right, and it sounds like this (
http://www.youtube.com/watch?v=hbGCeqgsDgQ&feature=related) then frankly I'll take the amplug. One usable tone on the move is better than zero and you can make a phone call while you're at it.
Actually I won't take the amplug either, I don't play on the move and if I did I'd just play acoustic.