For a developer, WooCommerce is brilliant. It's truly open and clearly designed for extensibility. The code is very well written and liberally commented and, best of all, there are hooks for everything. This means modifying behavior via my own plugin or theme functions rather than editing Woocommerce source, allowing the mods survive Woocommerce updates.
The paid-for extensions are great, but a tad pricey if you need a bunch! It would be great if Woothemes offered 'Club', 'Combo' or 'Bundle' options.