I'm very happy with the templating ability. The CSS is well written and commented and once you set up your own K2 style it's easy to get in and customize extensively.
Extra fields are a nice feature and combined with some CSS edits for the front end can make for very convenient data entry for your front end authors.
Tagging feature is also great and helps a lot for intelligent searches.
So far the ONLY really issue I have is that unlike systems such as VirtueMart, you CANNOT have the same item (product, article, etc.) in multiple categories. This is a big problem for catalog sites that want to reference the same piece of information in multiple locations for ease/method of search.
I really hope this feature can be added soon!