I'm not an expert, and I think I'm going to refer you to the Zamek article, because I think he discusses it a bit. I bought 50 pounds of the stuff, so it will be a while before I run out. Well, I don't recall having any, and some people swear by Gerstley, but I think I've seen people complain about running, pinholing, and occasionally crawling. Also unaccountable variability, which may be caused by variations in the composition or by small fluctuations in firing technique, or even perhaps by gremlins. If I can, I'll put a photo up here to illustrate pinholing, a problem I tend to have with rutile.
Crawling is something else entirely.
It seems to have to do with the surface tension of the glaze melt. If the surface tension is high enough, or if the bisqueware is dusty or oily when the glaze is applied, sometimes the glaze will pull back from some areas and leave them naked. There are even glazes that do this in an attractive manner, giving a rough and irregularly reticulated appearance, and are used by people on purpose. More typically, however, crawling results in cursing and smashing noises from the general direction of the potter. While I am definitely concerned with Gerstley, I am reasonably confident that we'll find adequate substitutes, or that I will, if necessary, learn to make my own frits.