I've been told a few times that Business Units in CRM 2011 are "tricky" and shouldn't be set up lightly since they have irreversible consequences for a CRM 2011 implementation.
On the other hand, teams in CRM 2011 seem much more flexible in managing record security.
For what reason would I still choose to set up Business Units in CRM 2011? What can I do with Business Units that I can't with Teams (and vice versa)?
Business Units are important for the security concept of Dynamics CRM. They define a kind of a boundary within you can define specific roles or permissions. They are also used to represent an organization structure.
Teams are used for ownership of a record (new feature in CRM 2011), which is handy if you can't define a single owner. They are also used for easier sharing - you could share a record with a team, instead of sharing it with multiple persons. Another usage is to grant permissions to multiple users with grouping them into a team and assign a security role to the team.