Just to add, we have been using StarShip (also by V-Technologies, very simialr to ShipGear, but with multiple interfaces available... FedEx, etc) for over a year now. The convenience of having the tracking # written back to Macola and also printed on the invoice is so nice! It saves digging through shipment manifests or bugging the shipping dept for tracking #'s. Plus we get fewer calls from customers looking for tracking information, etc. It also cuts down on shipping errors such as packages going to old addresses or shipping ground instead of next day, etc because all of that is only entered at order entry ...no need to rekey for shipping.
But it depends on what you are looking for a shipping program for, if UPS will provide you with WorldShip (they do so pretty freely I understand), there are some features built in that may provide what you are looking for without an additional shipping package. For instance, if you are considering ShipGear to be able to send email confirmations, WorldShip will automatically email confirmations as well. But it does not tie in with information in Macola, so if that is what is important to you (and saving time too), then check out ShipGear. We are using StarShip over terminal services, so the setup was a little more involved (but painless). Its working rather smoothly now.