I remember using several email accounts, before switching all communications to Gmail, however with this change I had to leave my emails and contacts from my other email accounts as it is.
However Gmail has now addressed this issue with a new feature which will allow users to import emails and contacts from their older accounts into Gmail.
The service to import emails is powered by TrueSwitch, a service provider that specializes in email migration.
The service has gone live and will be seen by select few users only right now. However Gmail will make it available to more users in near future.
You can find more information about the import feature by visiting the Official Google Help Page for importing data to Gmail.