We have installed a fair number of NIS 2003/2004/2005 on customer's machines over the past 2-1/2 years. The machines that seem to see the most grief are Win98 FE, and this is assuming a healthy OS/machine to start with. Although we have only been installing NIS 2005 a relatively short time in comparison, it doesn't seem to be any more problematic than the previous versions (knock on wood).
Most of the time, there seems to be some underlying problem with the MSI installer, script host or damaged Internet Exploder/system activex files that causes the installation to crater. One old Compaq Win98FE machine would run ok for a few days and then go back to the same problem - machine would hang on bootup. In the end, the customer bought a new machine and all was well after that.
Having said all of that, it seems that the NAV/NIS installer is weak - it doesn't identify potential problems, just goes ahead and tries to install, and sometimes it takes many hours to straighten out the mess, as it gets into a state where you can't reinstall or uninstall.
I agree with the poor marks given Symantec tech support. I haven't used the live person approach except for activation issues, since they want to charge for everything else. Their email support is pretty mediocre. There is a lot of info in their knowledge base, which is more than I can say for McAfee, but the search function is weak - too many false positives and no good way to filter.