The original iRig is pretty much just a fancy splitter, the iRig HD though has proper digital converters inside. The Sonic Port is great, only downsides are no input gain and no way to charge the device during use - both of which are possible with the new iRig HD2 by the way, which is where I would be looking.
Not a big fan of Positive Grid stuff though, too many annoying bugs such as pedals that just plain don't work (the Mutron!), knobs/switches that do nothing or work the wrong way round, etc. The amp models in Tonestack and Garageband sound better to me by far.