I'm using the monochrome mode of the 6300 (from the PS print plugin). I find the output to be very good. I have a spectro, so I used the QuadtoneRIP test charts and profile utilities to create monochrome ICC's for the papers I use. For glossy papers with a high DMax, you may get perfectly acceptable results without the profiling step.
But for matte papers, I find that without a profile or curve adjustment, the output of the monochrome printing mode is
too linear. That may sound counter-intuitive, since linearity is generally considered a good thing. But for matte papers, your black point is going to be at about L*=17. With black mapped to L*=17 and perfectly linear output from there to white, you end up with the mid-tones being considerably lighter than they should be. This results in weak-looking prints. It's quite easy to see this by printing
Keith's B/W Test Image on matte paper, first with the mono printing mode and no profile/curve, then again with an RGB profile and the regular RGB printing mode.
The QuadtoneRIP ICC profile will add a bit of compression to the shadows, to get the mid-tones back down closer to where they should be. The result is a much nicer looking print IMHO (and the shadows still retain more detail than they printing in RGB mode with an i1-generated color profile). So for matte papers I think the QTR profiling is important.
I prefer warm-toned prints, and that's quite easy with the mono print mode. If you prefer neutral prints, you may find that difficult to achieve with 3rd-party papers (although you should get good results with Canon's papers that have a specific media type setting). The problem with 3rd party papers is that the toning control is a simple x,y adjustment, it doesn't give you curve-based toning adjustments like QTR does. So you may not be able to get neutral results in the shadows
and the upper mid-tones, it really just depends on the paper in question. If I wanted to make neutral prints on 3rd-party papers, I'd probably use a custom RGB profile rather than the monochrome printing mode. The main downside is that shadow detail will suffer a bit (maybe not noticeable depending on the image).
It's a shame QuadtoneRIP only supports Epson printers, especially since it apparently won't be able to support newer Epsons starting with the 7900. AFAIK there's no technical hurdle preventing Canon support (after all, both QTR and TBW are based on the Gutenberg print engine from what I understand). TBW looks interesting, but I am a Windows guy so it's a non-starter for me.
