how sound is this joinery?  

You might be surprised.
Pinning seems unnecessary.
http://www.finewoodworking.com/SkillsAndTechniques/SkillsAndTechniquesAr...

it is essentially a but...
it is essentially a but joint with a dowel through. that joinery is sound with the right glue and clamping, with or without the dowel. a true mortise and tenon joint is much stronger but a lot more difficult to do. the dowel will ad an element of strength to the but joint but i must ask are you worried about the strength, construction or the look? I'm only asking because if you drilled a pilot hole using a bit with a 3/8" counter sink and got a good depth you could add a screw for strength and then cap the hole with a solid wood plug, sand smooth and that would offer a much better joint and give you the same look, actually from the look of it (seeing as the dowel doesn't appear to go all the way through the example photo) I have a feeling that's the method used for this piece. if your building it as a purist without the aid of screws that's a different story and an actual dowel may be what your looking for. Anyway, just a few thoughts. good luck with the project and i would love to see photos when your completed.

For a cabinet
That sort of joint is more than strong enough for a cabinet. Chairs bear the weight of a wiggly occupant testing their joints every which way. And that is before the occupant leans back on two feet and then slams the chair down onto all four again. A cabinet by comparison holds static weight up off the floor, no unexpected torsion or shearing applied to its joints at all. There really isn't an engineering case to be made for an extra strong joint in a cabinet. That you've asked on this forum of all places leads me to believe you might have an aesthetic or moral desire for joinery.

How about a pocket joint
You can buy an inexpensive jig to drill pocket holes where they won't be seen and then screw it together. Simple and strong. If you want the look of dowels, you can drill a shallow hole and insert a wood plug from the hardware store. It can be sanded flat or left rounded off and sticking out a bit like a button. Check out the kreg pocket jig.

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Great video, thanks tktoo. Indeed its quite good enough for a cabinet or tables and I like the exposed joinery, pocket holing etc isn't really my cup of tea.
I was just a bit concerned about twisting forces hence the pinning...but I like the look of it anyway, especially with contrasting dowell, a bit Mogensen like and easy to do.
Thanks for all the thoughts, it is more of a choice about exhibted craftsmanship I suppose, vanity maybe..but my hand cut joinery is finally getting good and its enjoyable to do.

Lap joints.
Even for such seemingly simple joinery, there can be many variations.
I'd love to see your results! Maybe it's time to resurrect the "Last Thing You Made" thread?
http://en.wikipedia.org/wiki/Lap_joint