Just recently I got to try out Guitar Rig, by Native Instruments, and it blew me away. Very flexible, and the amps sounded great. I only got to try it at a music store, mind you, but even in that environment it sounded good.
I Think the biggest issue people have with this is the time it can take to get a good sound. With an amp, you have Mic choice, and that's about it, but with software you can do almost anything. Very overwhelming. But, if you give it time, you can get some nice sounds. I have not used any other amp modeling plugins, so someone else will have to pitch in about them.