I love the iphone version. I have no lag whatsoever. I've never used Amplitube on my PC, but have used Native Instruments and a RockFrog.
Ever since I upgraded to Win7, I can't use them anymore due to latency issues. I've not found a solution yet.

Amplitube for the iphone is not cheap, not perfect, but it sure is great. I have recently been recuperating and the guitar and Amplitube as well as some great on-line lessons (GT!) have kept my sanity.

Paul