JamOrigin's MIDI Guitar 2 for iOS was just released on the App Store.
Announced as "almost a 1:1 port of the desktop/plugin version", JamOrigin's MIDI Guitar 2 App for iOS claims to make tracking very fast and more responsive/sensitive than most other (hardware) solutions - meaning it reportedly picks up articulations very well. However, it's still said to be slightly slower than the desktop version of MIDI Guitar 2 due to a fixed buffer size of 6 milliseconds - also iOS's audio processing may eventually be slower than a carefully configured audio desktop/laptop.
This version has most features of the desktop version, including pitch bends, better velocity handling, several guitar effects and even an amp sim with many Amp Cabinet responses. There is also a polyphonic tuner and interactive chord wheel - both free of charge. Some features are still missing of course, they are detailed here.
Yet, JamOrigin warns that MIDI Guitar is very CPU hungry - especially with amp and effects added. For using MIDI Guitars built-in synths and guitar amp and effects together, a new iOS Device (iPad Air, iPhone 6, iPod 6) appears necessary, but only using the MIDI output to play external synth in an external apps (including Garageband) should allow it to work on an iPad 4 or iPhone 5. Reportedly, MIDI Guitar's internal modules are more CPU-hungry because it will process everything on one CPU core, while playing an external synth allows to utilise both CPU cores and sustain higher loads. CPU efficiency will be worked on in the future.
The old In-App-Purchase translates over to the new version for previous users (all you need to do is to click "Restore" in the Shop to restore the old purchase). As for new customers, while the evaluation version (which includes many features such as a Polyphonic Tuner and interactive Chord Wheel) is free, the MIDI Outputs add-on must be purchased in-app for $19.99, and an additional $9.99 is necessary to unlock the new advanced features.