The most important features are:- Easy to set up for new users, even without knowledge of php or magento (or if you work in another CMS). - It's easy to customize your catalogue.- Price rules, notifications by email when products change price etc., make it easier than Magentos builtin modules manage our catalogues, no matter how big they grow! Nothing as such but we had problems because some customisation was not compatible between versions which makes upgrades difficult sometimes. For online shops needing flexibility about product pricing. We needed something able to handle very large ecommerce databases like many thousands items per day & prices changed often over time so this could also take care of VAT calculations automatically. The best thing about this software is it's flexibility, you don't need to have special skills or knowledge of programming in order for your store to run smoothly as well as look good! I dislike how some features are difficult to find/use at first (such as adding new products). It also takes time before everything looks exactly like what we want when setting up our site. We use ultracart because it makes managing inventory easier than other programs out there.