One of the brushes I use most for concepts and sketches is the Pencil Brush made by Koifishsushi on Deviantart. You can find and download the little awesome sucker HERE.
Above is the sketch and final painting I did for Schmidt for "Fühlzwerge". And even right now I use it for new sketches almost on a daily base. I like the smooth but traditional feeling of the brush a lot and the sketch tends to look more alive.
The only other -but real- pencil that gives me the same feeling is the Blackwing but being a leftie I tend to smudge it horribly and that's one of the reasons I use it only when I really think it's worth the hassle.
Nevertheless if you are into traditional sketches you might want to check out the Blackwing anyway. I have all three versions and can highly recommend these. If anybody is interested in sample pics let me know. :)
AND if you buy these anyway because they really *are* that awesome: Get yourself a long point pencil sharper as well. You will regret not having one at hand if you need one.