Based on this post I wanted to integrate Sowmyan's findings into the U2U CAML Query Builder. But I came to the conclusion that I can also update a user column in a list using the UpdateListItems method of the Lists.asmx web service with following query:
<Batch PreCalc='TRUE' OnError='Continue'>
<Method ID='1' Cmd='Update'>
I don't specify a field type and for the user column I only specify the ID of the user I want to assign the task to.
To retrieve rows using the GetListItems method I couldn't use the ID of the user but I can use his/her name:
<FieldRef Name='AssignedTo' />
<Value Type='User'>Karine Bosch</Value>
Luckily it doesn't always need to be complicated