Ah, interesting topic. :0
I use a .5 mechanical pencil, but mainly for sketches. The only time I used it for lineart besides quick doodles was when I had a lighbox and was tracing one of my sketches. Now, lightboxes are expensive. :P
Used .7 for a while and they aren't half bad. Fancy pencils are dandy, but they tend to smear... unless of course, you're shading or something and you want it to blend a little. *shrug*
Would just like to add that inking pens or even fine-tipped Sharpies are pretty good for lineart. Imo. Once you have it all drawn out 'n stuff.