I think the best approach is to make sure whatever service you decide to use has good import/export features.
A boss at a job I worked at a few years ago still keeps his contacts in an ancient version of Lotus organizer without VCard export support. As a result he's pretty much locked into it unless he ponies up $100 for the latest version of the software (released in 2003) which has VCard export. He's too cheap to do that so he'll have to stick with it as long as he is still on a version of Windows that supports 16bit software.
A boss at a job I worked at a few years ago still keeps his contacts in an ancient version of Lotus organizer without VCard export support. As a result he's pretty much locked into it unless he ponies up $100 for the latest version of the software (released in 2003) which has VCard export. He's too cheap to do that so he'll have to stick with it as long as he is still on a version of Windows that supports 16bit software.