I used kerneld + diald + PPP on a headless 4M 386 acting as router/masquerader
for a small LAN for about 8 months (until I got my lease line ;) ) without
any problems (excluding masquerading/kernel bugs). I added kerneld around
1.3.57. I also had an incomming mgetty+fax on it as well. The diald solution is
very stable, it is also memory efficent considering the filtering that one can
do.
If I needed demand ISDN, diald has more features than some dedicated ISDN
routers I have seen , like not letting things like OSPF trigger a dial but
still have it enabled for the other ports. A nice addition for diald would
be to bring the link up if some other port died, this could be useful for
ISDN backup for failed lease lines. I suppose you can do this now with scripts
but it would be nice if diald could monitor/manage multiple devices.