I've got SIGPro working across all common browsers and platforms (Mac and Windows) as part of my investigations into which gallery tool is best to use for a site I'm developing.
It's been rather an uphill struggle though, the sparse documentation doesn't help.
I did try editing the CSS in an attempt to get the thumbnails to display more evenly when they are disparate sizes, but doing so I ran into major cross-browser problems and had to revert to the default CSS file again. So I guess the developers have tried to cover all browser bases in their coding - and managed it pretty well.
Fritz, I took a look at your site and couldn't get the galleries to load in any browser.
I think overall I'll use SIGPro in certain instances, small galleries within articles for instance, but in general I'm leaning towards Gallery2 for the bulk of my gallery content.
You can see SIGPro in action on my development site
here
. Try it in different browsers and you'll see it works fine - IE6, IE7, Google Chrome, Firefox, and on Macs too.