I've had it quite badly three times in the past and the treatment I got depended on which GP I saw. First time saw a young locum who gave me anti-biotics which almost cleared it up.
Second time was a grizzled old guy who just told me to 'man-up' and manage it by doing useless breathing exercises and rinsing with salt water (which helped somewhat), although didn't help to cure it. It eventually came back and got so bad that I was having dizzy spells and almost had an accident driving home from work one night, so went back to the GP. Luckily I saw the young locum again and she got me an appointment with an ENT consultant who said it was really bad and gave me an extended course of anti-biotics, which eventually cleared it up.
I remember it was very painful and me feel very tired and unwell, I use to get really cross when I heard people say 'oh, it's only sinusitus, it's just a bad cold really' as it was very debillitating for me and was part of the reason I gave up work as I couldn't trust myself commuting.
These days I get occasional light bouts (alcohol sometimes seems to bring it on for some reason!) and find that Sudafed helps a lot, or Olbas oil is pretty good too.