I've done as detailed a comparison as possible between using MaxRoam and sticking with Vodafone (with Passport), which I'll share with you below. The idea with MaxRoam is that you buy a SIM (for around Euro 25) and use this instead of your normal SIM. Cubic Telecom offer very low prices - based I belive on them finding the cheapest routes from where you are standing to where you want to call. I assume this is mostly using VOIP, but I honestly don't know. An added benefit is that you can associate more than one number from more than one country to a single SIM. That means that I can (have, in fact) bought a SIM and attached an Irish and Italian number to it. So not only will it cost us less to roam, it'll cost our respective families in Cork and Cagliari less too.
Vodafone's passport program is a vast improvement on what they used to offer to travelers. If you go to a covered country (and use a Vodafone partner network), you pay a fixed charge for incoming calls regardless of how long you spend on the phone, and you pay that same charge on outgoing calls, plus a per-minute charge the same as you would get back home (i.e. if you weren't roaming). However Passport is only available on a monthly payment contract, doesn't cover SMSs and doesn't let you use the free minutes of your package.
I've tried to calculate how much we are likely to spend in Australia and China for starters, but those results can be extrapolated to the other destinations. I've put together three possible usage plans representing three possible patterns of use that we might put our mobile phones to while on the road. All the details can be read on this Google spreadsheet right here. They get progressively more intensive, and Usage Plan 2 is heavily geared towards SMS message sending.
First Australia - all prices in Euros:
Note that this assumes that I'm always on a Vodafone partner network.
China, where Passport is not available, is more telling:
In the case of China, Vodafone would appear to cost me more that twice MaxRoam's service. To see the details of the Usage Patterns, and the calculations on which I based these graphics, see the spreadsheet. Do let me know if you think I've done either option some injustice.
As a general disclaimer, I'd like to say that these calculations are as accurate as I need to make them in order to choose a telecoms solution. I am not offering them as a definitive guide to either service. See the respective sites for the full details.