I have a goal to be able to do as much as humanly possible with my iPhone, short of brain surgery. So I was pretty excited to try out the new AmpliTube software app that allows you to simulate amp, record, and send output to headphones or speakers.
There is a free version available as an app, but I went ahead and sprung $20 for the full version plus $10 for the 4-track recorder. Then you need a way to plug in your guitar. You can use the internal or external mic, or get something like the iRig, also made by IK Multimedia (
http://www.ikmultimedia.com/irig/features/). It's $39 and comes with a 1/8" plug and a jack for a headphone or speaker. All in all I think the price is a little steep--should include a full version of AmpliTube with the hardward IMO--but then you'd pay about this much for a cheap stompbox anyway.
On first look it works pretty well. It has 5 or so amp versions, plus 11 stompbox effects, plus a simple 4-track recorder w/ effects like reverb, equalizer, compressor. You can create and save tracks, merge 4 tracks into 1, add vocals, etc.
My concerns so far are that it's a fairly sizeable, power-intensive program, so you need at least a 3GS or better with a fair amount of extra space (so I've read), also the input device is a little awkward hanging from the phone. It's tricky to work the program on an iPhone with the small screen--I imagine it would be much better on the iPad--but it's not too difficult.
My brief experience with the sounds it produces is favorable. The amps and boxes tend toward exotic flangy and distorted effects (like most modeling amps) , but you can get a clean jazzy sound as well. So far I have not noticed latency to be a big problem.
I don't know if it will make you give up your Polytone or work as a reliable replacement for a gigging amp, but I foresee the day when a guy will go to a gig with his guitar and an iPhone or iPad and plug it directly into a soundsystem. For now it seems like a good substitute for an amp for practice or travel and a tool for making simple recordings.