I have a Deluo USB GPS and it works without an issue. As other alluded to, you can use just about any USB or Serial (with a Serial to USB adapter) and it will work because they are almost all NMEA compatable.
However, I do caution that GPS software on the Mac is more than lacking, it's downright laughable. Route 66 is by far the biggest piece of crud that I have installed on my Mac. I previously used Street Atlas 2003 (and still do in VPC, albeit slowly), and that just runs circles around Route 66. Things like breadcrumbs, voice navigation, voice control (Try zooming out of your map going 65 MPH with your mouse), even intuitive control with the mouse for easy zooming in and out. Split level maps are a necessity- one level for down to the street, another level to get the general feel of where you are at. And it does it all very quickly.
The problem is that Route 66 has no competition at all, and thus it'll never be improved beyond adding a few streets here and there. So, if all you're looking for is to see where you're at on the map, then go for it, otherwise, you may find that it's a bit lacking in features.