GSM is great, you'll have coverage in just about every country you pass through as long as you have a 900/1800 phone, but beware of the bills!!!
If you are outside of your home country, then both your home service provider and the local service provider in the country you are in really heavily mark up the charges for your phone call. This is called "roaming", which is pronounced "getting screwed".
By example, I have a Canadian-based GSM account, the local rate is equal to about USD 0.10 a minute. In Switzerland, the local rate for a Swiss provider (for Swiss people who have a local account) is also about USD 0.10 a minute. But, if I make a local call within Switzerland using my Canadian phone, I pay about USD 1.00 a minute. If I either make or receive a long distance call when I am in Switzerland, wow, the cost is about USD 2.50 a minute.
I'm just using Switzerland as an example, other countries are even worse. In Algeria, for example, I believe the roaming charge is USD 4.00 a minute.