You should also check Apogee Jam, very good audio quality and it works not just with your iPad/iPad but also with PC/Mac (http://www.apogeedigital.com/products/jam.php
I tried a bunch of app on the iPad and ended up with Amplitube.
- it emulates very well the dynamic of a tube amp,it will clean realistically when you rolloff the volume on your axe.
- the lowest latency of all the app I tried (even without engaging the ultra-low latency mode!)
- work well as a background application
- Slash package gives two very nice Marshall to play with
- Include basic multi tracking
- Crude metronome
- Basic multi tracking does NOT allow edition just punch-in (if somebody found a way please let me know)
- It can get expensive when you buy all the goodies
The POD. 2.0 was ok in its time (I still have one) but it does not have much dynamic and rolling off the volume on your guit will only yield a lower volume, muddier tone rather than a cleaner one. The POD 2 will more or less makes every guitar sound the same. Amplitube and other good modelling however will let your guitar sings through and provide tube dynamics.
P.S. I had a friend that had an iRig after he tried my Apogee Jam he got one as well!