1. For simple queries in the demo app everything seems to work well (though often exceedingly slow -- I haven't played too much with the optimizer, so I don't include that as an issue yet).  However, when I use a SQL editor (like universal sql editor or DTM SQL editor) queries don't return any data.  Very odd.  Te exact same queries work fine in the VB Demo application.
2. While I am able to access the database through Access and Startoffice's Base, even moderately complicated queries will cause the program to crash.
3. Every time I try to create a new connection to the datasource, I get a message that the file needs to be in single user mode.  If I then try to create the connection again immediately (no other changes to the environment) it connects no problem.  This happens mostly in the ODBC sql editing programs that I mentioned above.  Somehow Access seems to keep the connection information across sessions.

Most of your problems are most likely caused by doing something like:

Select * from InvoiceLine

which will export every invoice line in your QuickBooks company file, when you should be doing:

Select * from InvoiceLine where RefNumber='the invoice number'

for the actual invoice you're interested in, or:

Select * from InvoiceLine where CustomerRefFullName='full company name'

for all invoices raised for that customer.

MS Access datasheets, smart SQL tools and grids tend to all call QODBC whenever you you scroll through each line. By default this will run a QODBC optimizer update. Change your QODBC optimzer update operation to: "The start of every new connection (with "Load Updated Data" first)" instead.

See: How do I setup the QODBC Optimizer? Where are the Optimizer options?  for more infomration.

Using the Universal SQL Editor, try:

Select Count(*) from customer unoptimized

and

Select * from InvoiceLine unoptimized

instead.

Statements don't need to be complex, besides you are not dealing with a database here. What QODBC does is convert a SQL statement into a XML Request that's sent to the Intuit qbXML SDK and QuickBooks processes the request and a XML document returned that QODBC strips the tags from and returns just the data to you.

So a outer join over ten tables makes no sense ...... because there aren't ten tables, just one QuickBooks company file. QODBC provides a ODBC interface to QuickBooks and prehaps you need to view it for what it does. I've run thousands of test SQL statements that I tend to write up from scratch from all sort of stuff that gets posted here on the forum. An invalid statement will crash QODBC and leave the DLL in memory. After a crash, you must close the application you are using and re-establish a connection. Sometimes I've had to reboot my notebook. Crashing QODBC can corrupt the optimized file and overloading Intuit's qbXML SDK can damage a QuickBooks company file so it won't return data through the SDK interface.

So please make sure you have a backup of your company file. USE is not returning any data because the optimized .opt file is most likely corrupt. The QODBC optimizer is single user only. So you need to setup a different optimizer file location for different applications to use. To do this step up a seperate System DSN (see: How do I create or configure a QODBC DSN? ) and specify a fixed location of the optimizer database folder for each application to use (like: C:\Program Files\QODBC Driver for QuickBooks) instead of the default %UserProfile% method.