Yachting World has a feature about this in its Dec 2002 issue. Suggest you do a search in the liveaboard board too.
I've got Iridium which I've used without problem. Speed for data is very suspect though, making costs for that very high - maybe OK for text only. But for a pretty much go-anywhere voice 'mobile' I recommend it. Suggest you buy the phone from the US (eg ebay.com) as much cheaper but take care since there are lots of older style Motorolas about that cannot be upgraded for data and can only be used for voice. Newest models are about the same size as larger GSM mobiles but mine's a bit of a brick!