I have used the ISC card for some time now and I'm very happy with the overall quality of the card and its performance. I am currently testing it with TCP/IP and Novell IPX connecting to our Gandalf PRI equipment. Check out ISC's homepage and USR's homepage for more information on their products. I have heard un-confirmed reports that you can get the card for ~$350.
ISC has a few things that make this card unique:
It has the ability of automatically disconnecting/connecting DATA service when you pickup the analog phone
It supports Stac 4:1, Ascend 4:1, and Microsoft data compression
It supports both PAP and CHAP network authentication
It supports the IPCP/BNCP bridge protocol for supporting multiple users behind a single ISC ISDN card
It supports full IP and IPX source routing
It supports the NetBIOS/NetBEAUI protocol
PPP: Accessworks, Ascend, DigiBoard, Gandalf, KNX, Microsoft, and Motorola