For me the mail function is the real weak spot of Opencart. There is no way to track what admin emails have been sent, who they been sent to, or when they were sent. This makes marketing and follow-up customer service through opencart impossible. Sure, there are things like mailchimp that can help, but having a comprehensive native mail function that took care of these things would make a huge difference.
I'm surprised there hasn't been a mail module developed that handles these things--or maybe there has and I just haven't found it!
Anyway, other than that I find that it works reasonably well overall.