Hibernate many-to-many join table not populating

I want to associate two entity classes User and Unit via a many-to-many association using a join table. However this is not working as intended. First I'll briefly show you what I got so far.

Database:

|    User    |   |  UserToUnit   |   |   Unit    |
|Id|login|...|   |User_Id|Unit_Id|   |Id|name|...|

Entity User:

    @Entity
    @Table(name = "\"User\"")
    public class User implements Identifiable {
        public User() {
            units = new ArrayList<Unit>();
        }

        @Id
        @GeneratedValue(generator = "increment")
        @GenericGenerator(name = "increment", strategy = "guid")
        @Column(name = "Id", columnDefinition = "uniqueidentifier")
        private String id;

        @Transient
        @ManyToMany(fetch = FetchType.LAZY, cascade = CascadeType.ALL)
        @JoinTable(name = "UserToUnit",
            joinColumns = { @JoinColumn(name = "User_Id", nullable = false, updatable = false) },
            inverseJoinColumns =  { @JoinColumn(name = "Unit_Id", nullable = false, updatable = false) }
        )
        private Collection<Unit> units;
        [...]
        public Collection<Unit> getUnits() {
            return units;
        }

        public void setUnits(Collection<Unit> ous) {
            units= ous;
        }
    }

Entity Unit:

@Entity
@Table(name = "Unit")
@XmlRootElement
public class Unit implements Identifiable {

public Unit() {
    users = new ArrayList<User>();
}

@Id
@GeneratedValue(generator = "increment")
@GenericGenerator(name = "increment", strategy = "guid")
@Column(name = "Id", columnDefinition = "uniqueidentifier")
private String id;

@Transient
@ManyToMany(fetch = FetchType.LAZY, mappedBy = "units")
private Collection<User> users;
[...]
public Collection<User> getUsers() {
    return users;
}

public void setUsers(Collection<User> us) {
    users = us;
}

Usage:

user.getUnits().add(unit);
unit.getUsers().add(user);
unitDAO.saveOrUpdate(unit);
userDAO.saveOrUpdate(user);

But this does not add anything to the UserToUnit table. What am I missing here? Cheers, Chris