IS-IS questions

Hi. I asked on comp.dcom.sys.cisco a few days ago about the advantages of IS-IS over OSPF. Dave Katz replied that IS-IS has proven to be very extensible while still allowing for backward compatibility. IS-IS was also overhauled around the same time that a lot of ISPs were building their networks. My questions are these. When is a good time to use IS-IS? Should an ISP that wants to build a scalable network go with IS-IS right now? Or is there a certain point where IS-IS becomes desirable over OSPF? Is there ever such a point? I would think it is better to pick an IGP and stick with it. Thanks for any comments. -BD