I've messed around with a bunch & never liked any of them. They do get some use while tracking drums though... they're a great way to get rid of bleed for a jamming band. That's the one use where they're vastly superior to a full blown amp. But for tracking heavy guitars for real, I've yet to find a substitue. Perhaps the PODXT, some guys have got some really good sounds out of them, but then again, that's hardware.
Believe me, I really, really wish that someone would make a decent software amp sim. Even the Amplitube developers admitted to me that they're clueless when it comes to metal... I've worked with them a tiny bit, hopefully so they can develop something decent.
-0z-