A friend of mine has Amplitude. He raved about how great it sounded and how it was the best amp modeler he'd ever heard... blah, blah, blah.
I played around with it for an hour or so-- I didn't like it. It sounds OK. But a well mic'd amp sounds way better in my opinion.
If I was just doing some composing/arranging/songwriting and didn't want to mess with mic'ing my amp, Amplitude would be pretty handy. But I'd never use it as my sole guitar tone on a finished product. Now that I think about it, I'd just get a POD or equivalent since that won't use up any processing power in the computer. (I don't have any type of modeler, so that tells you where I stand on the modeling issue)