Are skip-the-line tickets worth it?

For the four or five genuinely mobbed sights, yes — everywhere else you're paying a markup for a queue that doesn't exist. Worth it: Vatican Museums, Colosseum, Uffizi, Accademia, Duomo climbs in summer. Not worth it: almost every other church, museum and archaeological site in the country. Be careful what you're buying: many "skip the line" listings are simply the official timed ticket resold with a 30–50% fee attached. Check the official site first, and only pay a reseller when the official allocation is gone or you actually want the guide that comes with it.