They partner with great providers - i.e. FusionBot search has been wonderful in creating a custom solution for our site.
Love the ease of 3dcart and the amazing customer support. Even if it's a little thing of not being able to locate something they are quick to help and go out of their way to do so.
I really love that I can customize my store according to my requirements and it's fully secure.
I was searching for a platform that had the bells and whistles which Yahoo merchant solutions missed so badly.
The only problem I have encountered is the inability to provide a coupon based on type of checkout option.
There is no way to make price adjustments without changing each one individually. It is repetitive and time consuming.