Hi,
I really like Abantecart, it's very good generally, but I did have an unpleasant experience trying to upgrade recently. I tried to upgrade from version 1.1.9 to 1.2 and I got the critical error processing message. No idea why but I did eventually work out how to restore the site. I downloaded the backup from /admin/system/backup and reuploaded via ftp, then opened phpadmin in cpanel and imported the sql file that was in the backup. I was very relieved to fix it but I'm not sure I'll be trying to upgrade it again which is a worry as I like to keep things up-to-date.
It's just a cautionary note really to anyone about to upgrade, things CAN go horribly wrong so make sure you only try to upgrade to the next version on from the one you have and do a proper backup through the admin. To be fair it does warn you to do that anyway before the upgrade.