I'm truly impressed with this industrial strength sitemap generator! Acrostic.me was able to use it to crawl our site and generate sitemaps (86 files, 600MB+) for over 5,000,000 pages . And as you can tell from the images it's had a MASSIVE impact on the amount and speed at Google is indexing our pages. You do have to be careful what settings you use when doing a very large site like this, and make sure you have plenty of RAM on the computer you're using!

Thanks for your kind testimonial

We will soon be releasing new versions where the memory usage has been further improved (a continous optimization process as the amount of data collected increases as new features get implemented), so the amount of URLs capacity in A1 Sitemap Generator should continue to climb in the future as well

For those doing large websites I can recommend checking:
http://www.microsystools.com/products/sitemap-generator/help/creating-sitemaps-large-websites/
http://www.microsystools.com/products/sitemap-generator/help/sitemap-generator-resume-scan/