I'm planning on buying an unlocked iPhone because I did my calculations based on my own usage and it seems to make sense.
I take very few calls on my cellphone. Most of my work week is spent at work and home; both locations have very reliable WiFi. I only give out my Google Voice number, so I use that for the bulk of my calls. Even at home, where I have no landline, I use Talkatone for outgoing voice calls on my iPod touch/iPad. Texting is free with Google Voice.
I figure my monthly cellular costs will be about $15-18 per month using an AT&T GoPhone pay-as-you-go prepaid SIM. If I buy an unlocked iPhone, my calculations say that I will break even at 11 months (versus the cheapest AT&T plan at $55 per month). Over a two-year period, my total cost of ownership should be about $500 less than a subsidized iPhone; plus the unlocked iPhone is worth more in the used handset marketplace.