Well, my kvass was interesting. Drinkable? Not really. But interesting. Let's write kvass-making off as a learning experience.
I filled two bottles with kvass last Monday morning and left them on a kitchen counter to start fermenting. When I got home I found one of the corks lying in a puddle of kvass next to the bottle. I stuffed the cork back in and let the kvass ferment for a few more days, but not surprisingly the kvass in that bottle was completely flat and tasted terrible. The second bottle, which remained corked the whole time, was closer to the real thing. The kvass was carbonated, smelled right but tasted oddly sour.
I'm not up on my food chemistry, so I'm not sure what happened. Did the recipe call for too little sugar? Was the bread at fault? Someone notes in the comments that you can't make kvass out of borodinsky rye bread, which is what I used.
For the sake of comparison, I made another batch out of kvass starter (photo above), one of the weirder things you can buy at the Russian store. Kvass starter is a thick, molasses-like liquid to which you add sugar, yeast and water. This fake kvass is sugary sweet, but more palatable than the sour batch.
My enthusiasm for making kvass has dimmed a little, although I may try again, using better bread.