You will be happy with mhelpdesk
First off, let me state that I have been trying to look for a good ticketing system to track repairs and invoicing for a long time. Most have all sucked. mhelpdesk is honestly the best thing I have come across so far. The community is great and the developers actually listen to your ideas and suggestions (via community support forum) and implement them.
Unlike ShopManager, mhelpdesk is very organized and well thought out. The interface is clean and relatively easy to navigate and its becoming more and more streamlined with each release.
mhelpdesk imports and exports estimate, invoices and payments into quickbooks, which is great for accounting purposes.
mhelpdesk is a little more expensive ($10) but really, you get what you pay for. What you pay in money, you make up for in time.
The one other thing that I have been looking for in a repair tracking system is an integrated POS. Currently mhelpdesk and ShopManager are both lacking in this area, however, mhelpdesk is integrating a payment gateway in one of their next releases (releases are pretty often).
Other than that, the support is good, the owner is down to earth and all around a cool guy to work with. I think you will be happy with mhelpdesk.