My organization is planning to migrate 10 site collections from Windows Sharepoint Services (WSS 3.0) to Sharepoint Online (2013) in Office 365. The largest of these site collections is under 2.5GB and all use the internal SQL Server database engine. Our WSS was installed on-premises and we will likely purchase a migration tool.
1) Can you recommend a Sharepoint migration tool you've had success with?
2) What problems or issues did you encounter during your migration, if any?
3) What are some of the migration pitfalls to be aware of?