Ah yes thats the method! I have the How To Paint Citadel Minis book and they call it feathering/blending in there. How smooth can you get the transitions? As smooth as wet blending or glazing/layering?
With practice, you can get pretty darn smooth. I'd say probably equal to glazing (at least, at my hand), and only marginally less so than wet blending. Wet blending will still give you the best blends, but it's probably the hardest technique to get down well. The good thing about the two-brush method is that it is doable even on smaller areas, where wet-blending or glazing isn't as practical--I even use it on teeth for crying out loud.
It's not a magic bullet, though. Like any technique, it requires practice and your first attempts won't be perfect. It is, I think, a useful technique to have in your toolbox, though, and can complement other techniques rather well. Unlike wet blending, it gives you a little more time to work on the model, since you can conceivably do the steps in different painting sessions. It's a little harder to do on large, smooth areas, and there I usually go for either wet blending or glazing/juicing.
Edit: And, just to make Supervike happy, I do still use my tweening technique every now and then.

That's a weird hybrid of layering and glazing. Basically, you get your two colors down, then thin each to a glaze on your palette and glaze color 1 up towards color 2, let that dry, then glaze color 2 down towards color 1. You basically wind up with very sharp lines at first, and soften the line via the glazing of the two colors.