I've heard that a lot of what is being sold as balsamic vinegar is artificially aged (ie, not in oak casks) and really poor quality compared to the expensive stuff. I've never really cared for balsamic vinegar, maybe I just haven't tasted a good one.
Our friend went to a tasting at the western Nebraska place, she had a sample of some balsamic vinegar that was priced at over $100 for a small bottle, she said it was unlike anything she'd ever tasted.
There are instructions for making balsamic vinegars in the Noma book, but it is a complicated process and even the Noma team wasn't sure how good their stuff would get over time.
My carrot and celery vinegars are interesting, so far I use the celery one more than I do the carrot one.