We recently had a national training event back in August and ISI was the training vendor for various courses... ethical hacking (CEH/CPTS), CISSP, and GIAC GCIH/GCFA.
The instructor for the CISSP course was great. The other two were lacking from my understanding. This was a very compressed timeframe, so it would certainly be difficult to cover two separate GIAC exams. The instructor for the ethical hacking course started the class off by saying "this class is supposed to cover material for CEH and [I believe] CPTS but will not prepare you for either exam." It was downhill from there.
But that's only my experience. Aside from their recent plagiarism, and the BackTrack "vulnerability" debacle, I have also heard good things about them as well.
The bottom line is just find out who the instructor will be ahead of time, and look into the background of that person.