i've used cakewalk's guitar tracks amp sim effect.
it works pretty good actually, but i've found that if u r just running line in. it still sounds "pingy" unless u add a little crunch w/ the sim. but then it just gets too gritty. i spent countless hrs tweaking it, but wasn't very successful.
maybe some of the others work better, but i just don't think u can really get a great guitar sound running line in....
just my opinion though.
-dj