ISAC was like a cult. if you were in, you made a typically very tight knit durable group of friends. I'd say the majority of students didn't want to do it and are glad that they didn't though. I'm one of them. I almost did it, but boy am I glad I didn't. Extra rent. Extra work. Terrible rates (like 12 bucks/hr or something as a TA iirc). I'm not sure it's worth its weight in effort on residency apps.
But if you're thinking about doing it, it's not the worse thing in the world. worse comes to worse, you lost like a month in the summer. It's not that bad. I just wouldn't do it.