I have been using WebAsyst Shop Script since 2011. The admin panel is top-notch — even a school kid can handle it. The engine works quickly, even with a large number of products; for example, I have 28,000 SKUs. The technical support is excellent; I always received answers to my questions. As for the downsides, I would note that the engine is not very convenient for search engines, although there are many SEO plugins in the marketplace — maybe I just haven't fully figured it out. Also, there's a downside to the subscription model that everyone is moving towards now! But perhaps that's a good thing — the developers will be able to focus on SEO!
