The only time I book a reservation through the 800 number is if it is something you can't do online ... like third-party award travel or something. The web sites have a huge amount of functionality now; it is great.
Word of advice for those who use Travelocity, Expedia, Orbitz, etc. Use those sites to shop around for good deals, but buy your tickets from the individual airline directly. You'll get the same fare from the airline website, but save yourself the $5 (or more!) processing fee they charge, plus most airlines offer bonus miles for buying from their website.