i have them, i've heard that people have had problems with them cracking, but that seemed to be few and far between, and less of a problem now then in the past. The only problem is i have them on the 96/98 heads, and the powerband got a little bigger, but no real power was gained. It really only seems to be beneficial to do headers on 99+ heads with a power adder.
Jim, i've tuned at higgins a few times and have seen many of their cars, what color is your gt? I saw the silver one with the SHM/blower combo, and a black gt with novi/intercooler combo, and was curious if either was yours