You can use ISDN speed but not all ISDN features.
See messages n° 215 on the old forums at Stanford.
As modem emulation you have a 64K speed, but no possibility for channel bundling...
We are all waiting for the drivers to use isdn4linux packages.
I use SuSE 6.2, kernel-2.2.10 and pcmcia-cs-3.1.0, and it works fine. Just remember to setup the correct parameters as said in message n° 215.