I'll post a little different perspective. I think probably the QB's are generally of very high quality. They are certainly better than the quality of my empennage which was a steep learning curve for me. Maybe better than my slowbuild wings too, since they were still part of that same earning curve.
That said, last summer at Oshkosh Van's brought an RV-7 quickbuild fuselage that really didn't look any better than my fuselage built from a slow-build kit. I was pleasantly surprised in that regard because I too had made mistakes in the past that, while successfully fixed, left me feeling like this plane-building thing was really tough. I'm definitely not saying I did it better, and I'm certainly no better than the average builder, but I think I've built my fuselage to quickbuild stage almost as well. I think that is a credit not to me but to Van's and his designs--that they can be built well by a reasonably inexperienced builder. Couple things I noticed on the quickbuilds:
1. Their riveting looked just just like mine--some pretty, some not so pretty, all structurally good.
2. They made some different choices than I would have. For example, one thing I didn't like was that they overlapped the side skin's conical bend over the center section bottom skin, rather than tucking it under the bottom skin which is cleaner looking and more aerodynamic. I asked one of the staffers there about it and they said that although the way they did it was different from what the plans indicate, it is easier for the quickbuilders to do it that way. Again, not a big deal, just different and less aesthetically pleasing in my opinion.
I often times hear people say that the quality of the quick builds can't be beat. Perhaps not by most builders, but I think by the time most builders get to this stage, they can probably get pretty close to matching the quality of the quickbuilds. Plus, as others have mentioned, some decisions have already been made for you which limits your opportunities if you go the quickbuild route. Also, for me, some of the most challenging of steps, and places where I have made mistakes, have come after passing the quickbuild stage. Thus, the quickbuild would not have helped me in those cases.
Bang for the buck, though, quickbuilds can't be beat. If I charged for my labor there's no way I could afford NOT to buy a quickbuild. Van's does a great job! Just go whichever way your time, money, interests, and personal satisfaction allow.