I, my self, am torn on the issue...
On one hand, I agree with Eric, simple - dial up a Marshall JTM45 or JCM800/900 and that's what the amp sounds like...
On the other hand, I agree with hubberjub, if you want a JCM, buy a JCM...

But the bigger issue that I think Eric is getting at is price, negating hubberjub's point, what normal, non-pro/non-wealthy person can aford all the the amps and guitars we want, right!? Modelers are inexpensive and made that way to get the most buyers possible while still making a quality product. A perfect example (there are many over the years) of a great idea without the follow-thru on total quality; the Crate V5 amp... Solid cabinet build, excellent choice of speaker size... Horrible speaker, bad circuit design (op amp distortion, no volume control).

As for the reccomendation of a Tech 21 over Line 6 ... All the way, bro, complete agreement there. Can't aford Tech 21, try Behringer GMX amps (though I don't like Jensen OEM speakers)... yeah, yeah, I know, digital effects, but analog "amp modeling".