We worked with Fusion years ago in conjunction with PFW. I believe the interface with 100 will be similar to that with PFW, at least in how it works.
Fusion POS has the ability to work entirely on it's own so it sync's a copy of the item master from 100 to Fusion. It operates stand alone because it was designed to handle large, multi-site implementations. (Think the Baltimore aquarium or an arena hosting an NBA or NFL sports franchise.) It supports a separate cash room for balancing a drawer against a Z tape away from the register.
The sync works bi-directional, pulling item, pricing, customer, and open AR invoice data from 100 and pushing transactions back to 100. Typically, you run the interface to only push closed register data. All cash customer sales are processed through a single sales order invoice so that inventory is properly decremented. Cash receipts are also processed so the invoice automatically clears. Sales to customers on account are posted individually to maintain the integrity of AR. Customers are allowed to make payments at the register against open invoices.
Overall, the Fusion POS system works well. I don't have experience with the old Hightower POS solution because our needs always involved registers that were not physically located on the same network. However I expect that the Fusion version is more feature rich but also more complex than the Hightower solution. Mike Gill is a stand up guy and Matt is very knowledgeable. We had similar responsiveness issues with support at times that @PamScott references above but we were able to eventually work our way through them.