We have done several of these. We used the Inaport tool each time.
That being said, it depends on what the customer wants to convert. Salesforce has a very bizarre, cryptic way of storing data. If the client only wants to convert companies, contacts, communications, it might be easier to dump to csv / excel and use the SageCRM utilities or MS SQL to load the data in.
It also depends on the volume of the data as well. Respond back or feel free to call if you have more questions - 312.513.9333.