A lot depends on the where and why.

I've found that voice calls on Iridium are ok despite drop outs, but trying to use it for data (modem) calls is virtually impossible. The dropouts occur because the Iridium is using multiple small satellites and you are being switched from sat to sat as they pass over.

While this simply causes bumps in voice calls, it often causes computers to drop connections.

If you are going to be anywhere in the eastern hemisphere, Thuraya is hands down the best service available.

They recently expanded service to include a lot of Africa and Asia.

They are cheaper to use, the handset is very portable and very easy to use, they have very good voice quality and service and as an added bonus their SMS service is integrated with the worldwide GSM system, which can save you some money on calls and also functions as a virtual answering machine, so you can recieve messages whenever you power up.

Check out this map

Inmarsat also offers a Regional-BGAN service that provides a high speed data connection via the Thuraya satellite in the same coverage area, at VERY reasonable prices.