Managing people's expectations is critical in anything you do in life, not just in Group Buys.
As we've seen in other GBs, nothing goes perfectly according to plan, but letting folks know about any issues, delays, etc., goes a LONG way in keeping things running smooth. Most people here have demonstrated that as long as they are aware of what's going on, then they are more than patient -- even if it's bad news.
Recent experiences with R4 and Sanctuary fresh in everyone's mind has many people on edge not to have a repeat of that experience. A key factor in those other GBs was the lack of communication, so it's understandable that when people start to see the same level of non-communication, they start reaching for their torches.
Personally, when I look back at monies that I payed in February and the OP status is still showing "Invoicing" with little or no updates, I start to grow concerned. I don't want a refund, I just really want to know what the next step is and how's it going.
You've taken on a huge responsibility, so I can only imagine the weight that you're carrying, but there are people at the ready to help share the load. GeekHack is a fantastic community of volunteers with a rich and varied set of talents. Whatever the issues are -- even money -- there are plenty of people that are willing to lend a hand and help out.
We all want the same thing -- this GB to be a huge success.