Here's my story... I started off as a zoom only kind of guy, really not seeing much value in prime lenses. Last year I began to feel like maybe "less is more" under certain circumstances. I hated always being the guy with a HUGE camera and HUGE lenses. At the time I had a 24-70 L that I loved dearly, my 70-200 f/2.8 L, and my 17-40 L. I decided that for a walk around lens, my 24-70 wasn't ideal and I wanted to try out a 24-105... turns out I really LOVED the 24-105. Awesome all around lens.
So I decided I would sell my 24-70 and try out primes for low light stuff. I bought my 28 f/1.8, 50 f/1.4 & 85 f/1.8. I LOVE using them at parties & bars, out with friends, leaving the flash at home, etc. 85 is an awesome portrait lens, or for anything where you want to isolate your subject. So low light, they're great, size wise, they're great--small & lightweight... I'm now trying to get myself using them more under normal conditions where light isn't so much the problem. So far I'm having fun with it. I don't think I'll ever give up my zoom lenses, but having one or two prime lenses can really open the door up to some new possibilities.
Sorry for the long post, hope it helps.
Tim