I never had any production stopping issues thus far needing support (on Zimbra 1 year so far), most of my support encounters are for functionality issues or bug requests, etc.
Are you properly categorizing the request, Zimbra really sticks to their SLAs related to the priority of things. If memory serves me right though, I think the evaluation support is not given a high priority in terms of response. With paid support you do have 1 hour SLA for critical 1 issues as well as immediate phone support with a support pin.
Zimbra does tend to have a variety of engineers in different time zones, usually if I have a noticeable delay I call the support line and have my ticket escalated to a better time zone engineer. A lot of them are California based which is 3 hours difference for us, haven't found it to be too big a deal.
Overall Zimbra Support is good, from a technical perspective they are very good and know the product well, I have noticed at times their response/SLAs are lost if they get too busy or back-logged. Generally though the product is solid enough that we haven't really needed support, couple that with running on RedHat Linux and Zimbra has been a very solid product for us.
Running Zimbra 7.2.3 on Red Hat 5 (64bit)