Well I had the original Peel 520 to try out, just for heck's sake... I tried it both on a 1st gen and 3rd gen iPod Touch, neither worked properly, it was tricky to setup in the first place, and the case actually scratched my old 1st gen's bezel... After I got everything set up I tried to make a call, it tried, rang then failed. I did this with a few numbers. Then, I tried to ring the iPod Touch, the 3rd gen (4.2.1) didn't ring at all, in fact, the 3rd didn't ring, receive or work at all, the 1st gen (3.1.3) when I rung it, the ringtone went off, but it remained on the home screen, so I couldn't answer it.
Even though I don't have an iPod Touch 4th Gen or the Peel II, I wouldn't recommend it simply for unreliability. I am now an iPhone owner and very happy 🙂