I'm new to CC4, but have been doing ecommerce since 1995.
So far, tech support has been great on our initial install.... they have helped sort out some issues with PayPal (the PTD must be on, not off, as it says in their knowledgebase). I've also found some layout bugs in Estelles templates, but she also helped me fix a bug with IE8 (hers not mine).
So far, I've been happy that somebody was there to reply to my "what the heck" stuff as I'm not a newbie (we own two full super powered servers in a class-A datacenter, so we be pretty hep on whassup with the normal kind of stuff).
Our setup is weirder than most because we sell services, mostly to companies, and the default mindset behind CubeCart is more mainstream for products to be shipped, and consumer stuff.
A few bugs have popped up, like I just ran into an issue with creating a long FAQs page, and the system times out if I try to save the page after a certain length (think 16 questions with long winded answers). It's also a bit wonky to have a "Company" option on the payment/registration page, but then not send that company name to the gateways, or print it on the email receipts (I managed to hack the printable receipt template, but the variables used on the HTML don't work on the emails...).
But you know, for the price, you can't beat it. Considering something like this used to cost $1500, and use all kinds of custom scripting languages, or annual licenses, or you could only host it on their server (not talking about CC) ... this has been pretty painless over the past month of on/off setup.
So.... depending on your skill level and expectations, it's worth trying out. :-)