The best "feeling" amp sim out there is for free. Acme Bar Gig makes it and they give it away.
http://www.acmebargig.com/product/shred/ I have
GR4 (and plan on getting the upgrade, Thanks for the tip!), Line 6
Gear Box Gold (Oldie but Goodie),
Amplitube 3 Free with quite a few add-on's from custom shop. Plus other free amp sims. And I have A/B'd these things side by side, and to me the closest thing to my tube amps (I have four different amps) is the ABG software. Don't believe me? Try it. Noof said.
Bothe GR and Amplitube do have strengths, each one different. I do like the Roland Jazz Chorus sound that GR4 has. And the Control Room of GR4 is really nice. But I will upgrade the GR4 to 5. I passed on the Amplitube deal.
Someone mentioned the lack of components with some of the available presets in Amplitube seeming like a "scam" I don't know if I would go that far, but it sure seems to me that it smells a little like that!!