Craft store selling around 20,000 line items. Moved from Magento 1 to Shopify and needed a theme which supported a large catalog. Pixel Union offer generally good support and even better if you pay for the Pro version of their Theme Updater app.
Easy to use theme. A bit pricier than some others but good for large catalogs and good support from the developers.
Set up myself. Some custom coding work for specific features done by Shopify partner developers.