I use lots of different brands, from Callebaut to Ghirardelli to Scharrfen Berger to even Bakers...
(typo: should read Scharffen Berger),
and I must say, I don't notice a great difference in the results. BUT... when it comes to semi-sweet and bittersweet, I'm pickier - For example, I never use Bakers, because I find the taste vile, but I also never use the higher end El Rey, because I find their stuff vile also - as always, it's a matter of personal taste. I make it a point to never bake with a chocolate I don't like out of hand, with unsweetened as the exception. No way, will I taste unsweetened out of hand! (although I did it once - blech).
Also, with semi-sweet and bittersweet, there will be different results, due to both the varying tastes and the varying cocoa percentages. For example, one bittersweet bar may be 80% cacao and another may be 60%. This would affect the texture of the finished product somewhat, not to mention the taste.
I've never used cocoa instead of unsweetened, but you can do so by substituting 1 oz unsweetened chocolate with 3 Tbsp cocoa plus 1 Tbsp unsalted butter. Although, the results will probably be a bit different.
A great book I highly recommend is "Bittersweet" by Alice Medrich. Not only are the recipes great, but she gives you an in depth breakdown of how to substitute one chocolate for another. Also how to sub with cocoa.
"Chocolate Goddess?" Well, thanks, I'm very flattered, but I think that title goes to Alice Medrich! Too bad she doesn't post here. smileys/smile.gif