It's really a matter of personal preference. I happen to like one of the styles, and having a semi-custom, form-fitting case is really nice. The ones I get tend to cost around $100, although I don't think I'd spend much more than that (certainly not $200). When they arrive in the mail, my first thought is usually, "I spent how much for this???", as the case looks like a few tiny pieces of leather; however, they do look really nice on the iPhone, IMO.
It'll be interesting to see what they come out with for the iPhone 4.
Contour Black Polycarb case...I've had one for the 1st gen and another for the 3G. I am sure they will come out with one for the 4. It is virtually indestructible.