Just came across this thread while digging through the archives and thought I'd share my experience:
A couple of years ago I did my spiced Christmas beer with bread yeast. The packet of ale yeast I put in originally didn't ferment, and I wasn't in a position to get any more brewer's yeast, so I made do with what was around the house. It actually turned out pretty well. Low attenuation, but some sweetness was welcome in that particular beer. There weren't any glaringly obvious off flavours.
This year I was making a similar spiced ale, and decided, for the sake of tradition, to do it with baker's yeast again. It was obviously a very different strain, but turned out okay too. It was much higher attenuation (OG 1.057, FG 1.012) and though it cleared up very nicely, it still had quite a bit of the "yeasty" smell that I associate with weissbiers. At bottling time it tasted pretty dreadful, and at two weeks post-bottling. But now, 5 weeks after bottling, it's drinking very nicely, especially with a slice of orange as a friend suggested.
Part of the reason for my relative success with baker's yeast is probably that these were both spiced ales and had pretty strong flavours and aromas coming from the spices (cinnamon, ginger, cloves) that masked any nasty phenols or other compounds that the bread yeast produced. I wouldn't make any other beers with bread yeast, but I may well continue doing the occasional spiced ale with it, just for fun.