When we try to void an invoice that was created through the S/R module it tells us that that feature is not yet available.  ??? Is anyone actually using the S/R module? It sounds so great, but, wow, is it buggy!

Not permitting voiding the invoice is not a bug (per se) but an intentional limitation.  
 
The concept of Service and repair is to enable the work order costs of the repair to pass directly to the invoice and not comingle with the average cost of the newly manufactured item of the same part number in Inventory.   Voiding a SR invoice would defeat that purpose because voiding invoices WILL put the parts on the invoice back into stock.  
 
Yes, we could probably develop an elegant program to reopen the repair work order and restore the costs from the invoice back to the work order but made a programming priority judgement not to.