CoolCart gets five stars because of its value. I've been using their service for 8-10 years with only a couple random & non-critical problems relating to presentation & functionality. I've had ZERO problems with anything involving money or security.
1) It's fairly crude regarding the presentation aspects. The user interface is clunky & outdated. Someone at CoolCart would benefit from taking a quick class in modern UI Design.
2) They could do a better job of marketing. I don't mean the sell-sell variety; I'd like to get occasional informative pings about what's new and why I should consider moving to the latest version. I turn my back on any hard-sell but I do appreciate the occasional gentle reminder.
1) It's solid and it works.
2) It's secure. They stay on top of PCI compliance and what's happening regarding security. Their server certificate is VeriSign Class 3 128-bit/TLS, which is plenty good enough for me.
3) They are responsive
Several months ago my bank (Bank of America) was rooting around and said that my site might not be PCI compliant. I told them that CoolCart managed our transactions and BofA should check with CoolCart. The response from BofA, a few days later, was "you're good."
If you want to control everything regarding your users' experiences, you should be willing to buy/write a high-end cart on a secure server, and know how to work with and pay for, that structure.
I own a software biz and there's plenty of talent here that could do eCommerce from scratch but that's not our main line of business. If you want to roll your own, think about the costs & overhead. I've found that CoolCart had been good for my business.