Amsoil is made in Superior, Wisconsin, so we hear quite a bit about them here in MN. Their sales pitch is a little smarmy, but it is a good product. I'd say no better, no worse than Mobil 1.
I've used both brands and changed oil, gearbox and differential fluids when I've purchased cars with anywhere from 20K-60K miles them. The cars previously had used petroleum products and I have had no seal leaks with the switches to synthetic. However, I have heard similar stories like Mr. Wood has mentioned. I'd be curious to know what percentage of folks who switch to synthetic have seal problems.