I'm almost exclusively in group #3. Once in a while I'll use Travelocity or Orbitz to get an overview of prices, but almost always I just watch fares at united.com and book there. Since I fly UA unless it doesn't take me somewhere (which is rare), I stick to that Web site. I get 250 mypoints, extra EQMs for paying with UA Visa online, and UA booking bonus. So why book elsewhere?