Yes, if you choose Open Source, you get the community. Community is of course a grat thing, but the fact is they don't have to help you. If you really get into trouble, you stay alone with your broken code and your malfunctioning shop.
Companies that offer paid solutions have quality support. If you bought the shop script from them, they HAVE TO help you get it working. And they usually can do it quickly because it's them who written all the code, and not the abstract "community".
So I was looking for paid solutions only. I tested about five of them and stopped at Avactis
shopping cart - it's not that cheap, while has very nice customization features. And the admin area is MUCH clearer that of Open Source stuff (especially Zen Cart).
And it integrates with Google Checkout, too
Here's one more helpful link:
Shopping Cart Reviews - there's quite a lot of stuff to choose from.