I've been looking for a good shopping cart to use with Joomla. I used the Phoca Gallery plugins to display pictures and found it to work quite well so I decided to give the Phoca Cart plugin a try. It's easy to install and setup and is quite feature rich so it didn't take me very long to get my Phoca Cart up and running. Their setup wizard works well, walking you through setting up the minimum required settings to get your cart up and running. After using it a bit though I have a couple issues with the shipping module. You can restrict the different shipping options based on things like amounts, country, region, weight, size etc. and that all seems to work well. If you're using it in the US though, it can be cumbersome and lacks some needed features in my opinion.
The first issue I've run into and have yet to solve is being able to enable "USPS Media Mail" shipping methods for items in a particular category. Let's say you have a "Books" category like I do. I would only want Media Mail shipping options to be shown for items in the books category and not for any other categories. There doesn't seem to be a way to do this in it's current state as a category rule simply doesn't exist.
Another issue I have with it is there doesn't seem to be a plugin that will interface with the USPS, UPS, or FED-EX to automatically calculate postage. This would obviously make things a lot easier when it comes to calculating shipping costs. As it is now, let's say you have a USPS chart with media mail prices, you'd have to create shipping options for each weight range and this can be tons of manual entry. An import/export for shipping methods would be very useful. That way people could save time and effort if they had to setup multiple sites.
Phoca Cart is a very capable shopping cart. It's easy to setup and use with a nice user interface on the back end. For it to be useful to me though I think the shipping methods part of the application would need some work.