I can take my unlocked GSM/3G phone to a foreign country, buy a local prepaid SIM card for less than €5, and save a fortune on roaming charges. Operator competitiveness is enhanced when I'm not abroad, since they know I can dump them any time - so long as I didn't take a 'free' phone with a long contract - and go with their competitor. I pay €9 a month in Finland for a phone line, no included minutes/sms, and uncapped 384kbps data usage, on a rolling one month contract.
I rarely travel outside of the US. Currently I do use an unlocked GSM phone, but only because it is very well integrated with voip and I don't have a voice plan.
I think there is a little confusion here. CDMA is a technology, GSM is a standard. So its not comparing apples to apples. The right comparison is between TDMA and CDMA or between GSM and CDMA 2000. From a technology viewpoint, most people would agree that CDMA has clear advantages, which is why GSM's 3G variant is based on CDMA. From a standard's point of view, GSM wins because it has things like SIM cards and widespread availability leading to economies of scale. When it comes down tp picking a particular operator and price plan, that has more to do with local regulation and carrier marketing than the technology or the standard. Near me Verizon has the best network because they put up the most base stations. T-Mobile is the cheapest because they have to compete with the others harder as they are the smallest.
I have the Sprint Touch Pro 2...it is CDMA and GSM all in one. Abroad, I can pop in a prepaid SIM for cheap calls or use Sprint's SIM if I want unlimited data. A great compromise.