I really love using brushes to ink, but carrying acrylic ink, brushes, cleaner, water and all that jazz doesn't make sense for just sketches. The first pen brush I used was the Micron brush pen, and it really sucked. The brush started out stiff, and after a few uses the point split and wouldn't draw smooth or thin lines again. Also tried using water brush pens, both filling the chamber with ink and leaving it empty. Filling the chamber just caused the thing to clog, probably due to using acrylic ink. Naomi Romero suggested these Pilot pocket brush pens (
http://tinyurl.com/jmd7hpx) and it has been working really well so far! No bleeding problems on regular sulphite drawing paper or bristol.
As for use, it takes a bit of getting used to, since you're holding it like a pen, but it acts like a brush (obvious point, I guess). When I'm using it, I'm moving my whole hand/arm, instead of resting the hand on the paper and just rotating the wrist. They're good if you like exaggerated linework and going hog wild with varying thickness. If you want exact control over line width, pens are still the best. Like any tool, it just takes practice to get the feel for it. Also, you're still going to be swapping out brushes. After a certain size the line variance starts to look 'wrong', so you swap up to a larger brush. The same thing happens in digital inking, now that I think on it.
I really loathe to say things like this, but in this case the brush quality does matter. No one's saying you have to get sable brushes, but high enough quality that they keep a point. Keeping ink out of the ferrule, cleaning after use, and reshaping the bristles will help make brushes last. Master's brush cleaner is amazing for this. Sorry if I'm repeating things you already know!