Is Roofing Underlayment Unnecessary?

[ThingOfBeauty]

Not much to add except I recently got to see some 20-year-old-ish GAF stuff and it was in terrible shape. Brittle and cracked. I'd stick to regular felt if intending it to do anything.
I also agree with using ice guard under metal. Metal leaks.

[hank mavery]

Some of the synthetics claim that, if installed properly, they can be leak-free with no shingles above for up to 60 days.

[sroskens]

Felt will dry and crack over time under the shingles. It's only value was as short term protection and a vapor barrier. Remember moisture flows both ways.I am a big fan of the synthetics and can attest if properly installed they will stay leak free. You have to use caps to install them to prevent tear-off and seal around the nail/staple. They are only limited by their UV rating and some are up to a year now. Get the best quality and you'll pay about twice per square as 30# felt. We've all tacked our felt down only to come back the next day and see a bubbly mess. Not with the synthetics. Four feet wide and much lighter.

[hank mavery]

If the caps prevent leaks, what prevents the leaks where the roofing nails penetrate? Is it the material of the asphalt shingle being compressed by the roofing nail?