I think I am quite OC about guitar tone. Here are some of my experiences with software:
Guitar Rig - great emulations, but if you have a not-so-nice sound card, you might deal with latency issues. The finger response is so flat, the high gain sounds are so noisy... not the hum type of noise but the problem with amplifying decimation noise. It's that SHHHHHHHHHH sound you get from digital multi-fx, only more apparent. Use ASIO for best results. But what I like is the number of fx you can put, and I loved the synth emulations best. But for distortion, forget it... it shouldn't be a problem to use distortion pedals in front of the PC, except if you want to track the guitar dry and later on send it for reamping.
Amplitube - I prefer GR to this, and I don't recommend it at all. The emulations are not as good as GR, and same noise + latency issues. Use ASIO for best results as well.
I do have to admit that versatility-wise, modellers do the job well.