It's so hard to find the perfect case! Back when I had my 3GS I probably went through 5-6 cases in 16 months before settling on using my phone without one.
Now that I've stepped up to a 32gb 4S I'm trying to use the same case until it breaks. Cases can cost anywhere from a few dollars to almost $100 if you're buying those super fancy aluminum bumpers some people have shown off on the forums. I'm sure the some of case-obsessed could've spent more on accessories (skins, wraps, light mods, etc.) than the cost of their iPhone!
My InCase Bird's Nest case has held up since November, but it's starting to look rough around the edges! The OCD in me wants to replace it based on it's looks, but I'm doing my best to resist!