In terms of preparing students for the work place, I worry about always ensuring you've got a good balance of students in each group (leaving aside issues of having too many of one 'type' in a class) - as at times individuals will have to perform many different roles - and may be in very unbalanced groups.
I tend to vary between directed groups, random ones & self selected. I think it depends on the nature of the task; the proportion of marks they're going to get for the group element vs. the individual element of the task, how long the task is going to take etc.