It's better to not use any kind of stop leak, but I believe the Alumaseal stuff is the lesser of the evils'. At least the Alumaseal will help conduct heat, where as the other goo's will decrease the rad's ability to transfer heat.
I put Alumaseal in the Dak radiator about 5 years ago because the lot I bought it off of smashed the power steering pump into the rad and created a pinhole leak. Knock on wood, it has never leaked a drop since and I have never been able to cause it to overheat (Even here in Phoenix when it gets to 118 deg outside) and as far as I can tell, the Alumaseal has not caused any problems. Who knows, it might increase the wear on the water pump or something but I have had no issues. I will probably sell the Dak soon, so the $5 for the Alumaseal was much cheaper than replacing the Radiator.